Saltwater Aquarium Chemistry: How To Supplement Calcium And Alkalinity

by | Feb 15, 2003 | 0 comments

There is no aspect of reef tank chemistry more important than calcium1 and alkalinity.2 Many of my previous articles have described various aspect of these systems in detail. In reading those articles, aquarists will note one pervasive theme: that maintaining appropriate levels of each is very important. Moreover, the easiest way to ensure that things do not go seriously wrong in adding these to the tank is to use additives that have balanced amounts of calcium and alkalinity. 3, 4 For purposes of this article, a balanced calcium and alkalinity additive is one that provides calcium and alkalinity in proportions that match that used in calcification to form calcium carbonate. Using this type of additive typically prevents overdosing (or underdosing) of either of these two relative to the other.

Independent (unbalanced) additions of calcium and alkalinity do have significant uses in reef tanks. These uses include correcting existing imbalances5 between calcium and alkalinity and in “fixing” a starting salt mix that might not fit the calcium and alkalinity values that an aquarist desires. While the regular use of such additives can work well, it frequently results in substantial imbalances between calcium and alkalinity. In a perfect world, with perfect test kits used perfectly and frequently by every aquarist, such additions would work out fine. More typically, however, they lead to imbalances. In some cases, the imbalances are extreme, such as calcium levels less than half of that in natural seawater (a serious condition for calcifying organisms6).

It is for this reason that I strongly encourage aquarists to select a balanced calcium and alkalinity supplementation scheme. There are, however, many such schemes to choose from. This article will provide the information necessary for aquarists to choose an appropriate scheme for their specific needs. Such deciding factors as cost, complexity, impurities, and a variety of others things come into play in making a decision. Towards the end of the article are two tables, one that outlines the cost aspects of each system, and one that summarizes some of the other differences. In the end, I don’t pick any one of these schemes as being best for all tanks, though I do indicate what types of tanks each system works well for, and what types they don’t. For experienced aquarists, that will be all they need to make informed choices. For beginners, I’ve also included some guidelines at the end of the article that should help them integrate these various concerns and point them in the right
direction for the type of tank that they are considering.

Aquarists should also not be averse to combining two or more of these schemes. In some cases there are substantial synergies that can be obtained from combining systems. Some of the more common combinations are discussed below.

I will say that I do not believe that there are any other systems commonly used that are as good as these detailed here. So these choices should cover the systems that people ought to consider unless they have very peculiar situations (or something new is invented in the future).

I’ll also try to straighten out some misunderstandings that aquarists frequently have about them (e.g., issues around heavy metals, either added intentionally or present as impurities). I won’t, however, have space in this article to give exacting details about how each is to be used. In many cases, there are existing papers describing them.

The systems to be covered in this article are:

  1. Limewater (aka kalkwasser), used in a reactor or not, and with vinegar or not
  2. Calcium carbonate/carbon dioxide reactors (CaCO3/CO2 reactors)
  3. Calcium carbonate used without a reactor
  4. Calcium acetate
  5. One part inorganic salt mixtures
  6. Two-part liquid additive systems
  7. Water changes

Some of the pros and cons to be discussed include:

  1. Typical initial and operating costs
  2. Limits to the amount of calcium and alkalinity that can be added
  3. Amount of work for the aquarist
  4. Space required around the tank
  5. Effects on pH
  6. Effects on phosphate (delivery or removal)
  7. Delivery of impurities to the tank
  8. Risks from overdosing
  9. Any human safety concerns



Limewater (also known by the German term kalkwasser) has been used very successfully by aquarists for a number of years, and it is the system that I use on my tank. It is comprised of an aqueous solution of calcium and hydroxide ions that can be made by dissolving either quicklime (calcium oxide, CaO) or lime (calcium hydroxide, Ca(OH)2). The only inherent difference between the two is that if you add a molecule of water to quicklime, you get lime, and that a great quantity of heat can be generated when that happens.

CaO + H2O → Ca(OH)2

Quicklime + Water → Lime

Consequently, dissolving quicklime can make water quite warm, especially if an excess of solids are added.

The calcium ions in the solution obviously supply calcium to the tank, and the hydroxide ions supply alkalinity. Hydroxide itself provides alkalinity (both by definition and as measured with an alkalinity test), but corals consume alkalinity as bicarbonate6, not hydroxide. Fortunately, when limewater is used in a reef tank, it quickly combines with atmospheric and in- tank CO2 and bicarbonate to form bicarbonate and carbonate:

OH + CO2 → HCO3

OH + HCO3 → CO32- + H2O

Once in the tank at an acceptable pH, there is no concern that the alkalinity provided by limewater is any different than any other carbonate alkalinity supplement. The hydroxide immediately disappears into the bicarbonate/carbonate system. In other words, the amount of hydroxide present in tank water is really only a function of pH (regardless of what has been added), and at any pH below 9, it is an insignificant factor in alkalinity tests (much less than 0.1 meq/L). Consequently, the fact that alkalinity is initially supplied as hydroxide in not to be viewed as problematic, except as it impacts pH (see below).

The fact that limewater is very basic (the pH is typically above 12) demands that the limewater be added slowly to a tank unless very small additions are made. The reason for slow addition is two-fold: to prevent the local pH in the area of the addition from rising too high (slow addition permits more rapid mixing with tank water to reduce the pH), and to prevent the overall tank pH from rising too high (slow addition allows the tank to pull in CO2 from the atmosphere during the slow addition, mitigating the pH rise). Some aquarists advocate rapid addition,9 and that is fine for additions that would add less than 0.2 meq/L of alkalinity to the tank, but an addition of 0.5 meq/L (the equivalent of adding 1.2 % of the tank volume in saturated limewater or 14 grams of calcium hydroxide into a 100-gallon tank) drives the pH of the whole tank too high (up
by about 0.5 pH units

Consequently, limewater is most often added slowly, by dripping or slow pumping. Often it is added as the top off water, replacing most or all of the evaporated water. The pumps add cost and complexity to the system, especially if combined with a float valve or switch (I use the latter and a Reef Filler pump).

As mentioned, limewater has a very high pH. This high pH can have significant advantages with respect to impurities present in the lime. Phosphate and many heavy metals will precipitate, either as calcium salts, or as metal oxides and hydroxides. Copper, for example, has been suggested by Ron Shimek to be a concern in reef tanks.11, 12 Copper hydroxide is very insoluble in limewater because of all of the hydroxide around. From an aquarist’s perspective, there will simply be none in clear limewater assuming that it has been given a chance to settle out because copper hydroxide is so very insoluble. Many aquarists get colored residues in limewater systems, and these colors are coming from metal impurities that did not get into the tank. If it turns out that copper additions to reef tanks are best avoided, then using limewater may be the best way to supplement calcium and alkalinity because most of the other
systems do not have this self purification mechanism.

Another advantage of limewater may be it’s ability to reduce the phosphate already in the tank water.13 While it may be as simple as precipitation of calcium phosphate where the high pH, high calcium limewater meets the tank water, the mechanism and extent of this effect in typical reef tanks has not been established.

Another important consideration for limewater is the upper limit to the amount that can be added to a tank. If an aquarist has a tank near the high end of calcium and alkalinity demand, then replacing all of the evaporated water with saturated limewater may not be adequate. There are a couple of tricks to get a little more from the limewater. These are adding fans to increase evaporation, and adding vinegar to increase the solubility of the lime in the water. Both of these systems have been successfully employed by many aquarists. Additionally, the use of a small amount of one of the other balanced additive systems (especially the two-part additive systems) is often used by aquarists give a little boost to tanks that need a small amount of extra calcium and alkalinity beyond what limewater can supply, without incurring significant capital costs. Likewise, they can be successfully combined with limewater during periods of low evaporation. Unlike some other supplementation schemes, tank salinity will not increase over time through the use of limewater.

The cost of a limewater system can range from very little to quite a lot. If one uses an inexpensive drip system ($20) and bulk sources of lime (like Mississippi Lime Company that doesn’t sell to individual aquarists but has sold large quantities to stores), the cost can be almost inconsequential. The Mississippi Lime Company quicklime that I use cost me less than $0.50 per pound as part of a large bulk purchase. In such a system, the cost per thousand milliequivalents (meq) of alkalinity is on the order of $0.03. I realize that this number means nothing to most aquarists, but I’ll use it to permit cost comparisons of very different supplementation schemes, and at the end of the article, I’ll convert it to yearly costs for some typical tanks. Hobby and lab grades of calcium hydroxide
will be more expensive. A pound of calcium hydroxide from Two Little Fishes costs about $13.50, or $1.10 per thousand meq of alkalinity.

Of course, dosing pumps can be several hundred dollars, a good float switch can be $50-100, and one needs to get a reservoir as well (often a plastic container like a trash can; I use a 44-gallon Rubbermaid Brute trash can). Depending on the setup, the limewater reservoir can be far from the tank; even in another room or on another floor of the home. A pump like a Reef Filler pump can be used to send the limewater significant distances, freeing up space around the tank.

Some people use Nilsen reactors to deliver limewater. These systems automate the delivery of limewater to the tank, and, of course, the costs rise. They consist of a chamber where fresh water enters, is mixed with solid lime, and the fluid limewater exits the system and travels to the tank. They do not permit any additional calcium or alkalinity to be delivered to a tank compared to other limewater delivery methods (assuming that both use saturated limewater), but many claim them to be less hassle than delivery from a still reservoir. Addition of limewater with the simplest drippers may require daily attention, while delivery from a large reservoir may require attention only once every 3 weeks, which is about the same as typical Nilsen reactors. All of the other comments about limewater apply equally well when used with a reactor, a dripper, or a slow pump from a still reservoir (except that the vinegar/limewater combination is technically difficult to use with a Nilsen reactor).

On the negative side, limewater does have some concerns that don’t apply to most other systems. One is the effect of overdosing. All calcium and alkalinity additives, if added in sufficient overdose, can case abiotic precipitation of calcium carbonate in the tank. Limewater, however, is especially prone to this effect for two reasons. If overdosed, the high pH of the limewater will rapidly convert much of the bicarbonate in the tank to carbonate, increasing the likelihood of precipitating calcium carbonate. Also, addition of solid lime particles can cause local extreme spikes in pH and calcium that nucleate precipitation of calcium carbonate. Consequently, limewater overdose, especially dosing of lime solids, is by far the most frequent cause of “snowstorm” events where calcium carbonate precipitates out all through the water column. In some cases, the tank can look like milk. The good news is that this event rarely causes lasting harm to tank inhabitants (at least that has been recorded to date), but it is nearly always upsetting to the aquarist.

One final note on lime: The high pH of the liquid and the dust hazard of the solid are not to be treated lightly. Inhalation of the dust is to be avoided. Splashing of limewater onto skin is also to be avoided, and should be followed by extensive rinsing with tap water if it happens. Splashing of limewater into the eyes is especially to be avoided, and the use of safety goggles when using large amounts or in situations where exposure is likely is prudent. Extensive and immediate rinsing with tap water, followed by professional help would be advised in the case of eye exposure.

Calcium Carbonate/Carbon Dioxide Reactors

Calcium carbonate/carbon dioxide reactors work by removing water from the tank, adding carbon dioxide to reduce the pH, and then allowing the more acidic water to dissolve solid calcium carbonate media that is present in a mixing chamber. The water is then returned to the tank with its extra calcium and alkalinity (bicarbonate):

CaCO3 + H+ → Ca2+ + HCO3

Reef tanks employing such reactors typically run at a pH below that of natural seawater, with typical tank pH values of 7.7 to 8.1. The reason for the low pH is the constant delivery of low pH solution to the tank, adding both excess CO2 and bicarbonate. Tanks then blow off this extra CO2 and the pH rises, but the effect is typically not complete, and the pH stays below what would be the case if the same tank water were fully aerated (that is, equilibrated) with normal air.

The media used is important in these systems, with the aragonite form of calcium carbonate being more readily dissolved than the calcite form. Also the nature of the impurities can be very important, as nearly all of the impurities will be dissolved and delivered to the tank. Some of these impurities may be desired by the aquarist (such as magnesium and strontium) and some may not be (such as phosphate or copper). Phosphate seems to have become a point of competition between commercial suppliers of media for such reactors, though I would advise aquarists to be skeptical of some of these claims. Using CaCO3/CO2 reactors has been shown by Simon Huntington to provide a substantial input of copper to his system (0.229 parts per billion of copper per day). In his case, it is substantially larger than his input from foods (0.0235 ppb of copper per day).

One big advantage of these reactors is that they can be scaled to deliver any amount of calcium and alkalinity needed by any tank. For this reason, they are greatly favored by those who have tanks with a high demand for calcium and alkalinity. Because of the low pH that often results, many of these aquarists choose to dose limewater in conjunction with the reactor, not because the reactor cannot supply enough calcium and alkalinity, but purely to raise the pH in the tank itself. The synergy between limewater and CaCO3/CO2 reactors involves more than just pH. Limewater uses up CO2 and CaCO3/CO2 reactors deliver it to the tank. Together, they combine to keep CO2 (and consequently, pH) more in line with natural seawater.

Calcium carbonate/carbon dioxide reactors take up a substantial amount of space, since one needs a carbon dioxide cylinder, a reaction chamber, and a pump. Typically, these systems are used close to a tank, but they could be remote if appropriate water flows to and from the tank could be worked out.

Once an aquarist has properly adjusted the reactor system, it requires minimal monitoring for a substantial period. Tank salinity will not increase over time using calcium carbonate/carbon dioxide reactors.

The likelihood of problems from overdosing using such a reactor is minimal. Since the pH is typically low, even substantially elevated calcium and alkalinity values may not cause a dramatic calcium carbonate precipitation event. More likely is just slow precipitation onto heaters and pump impellers. Accidental delivery of large amounts of CO2 to the tank is a concern, but that is a rare accident.

The initial costs of such reactor systems can be considerable, typically about $300-500 for the reactor itself, and another $75-150 for the CO2 apparatus. Media costs vary, but $20 for eight pounds is typical. That puts the media cost at $0.28 per thousand meq of alkalinity. The carbon dioxide cost also needs to be figured in, so that might push the total to about $0.35 per thousand meq of alkalinity.

The primary safety concern for these systems involves the carbon dioxide gas cylinder. Any high-pressure gas cylinder can be very dangerous if the cylinder head should become damaged. So be careful to not drop such cylinders least they become rockets.

Calcium Carbonate without a Reactor

In a recent article I described in detail what one could do with calcium carbonate when not used in a reactor. In my opinion, the best use is to dissolve the calcium carbonate in fresh water, and use it as the top off water for the system. Other uses, such as adding particulate or milky products directly to the tank seem like poor practice to me (since particulate calcium carbonate likely won’t dissolve in a reef tank and may actually nucleate precipitation of additional calcium and magnesium carbonate from the water).

The big drawback to this method is that not much calcium carbonate will dissolve in fresh water, regardless of what form the material takes (including fine aragonite particles). One is limited to about 30-ppm calcium in such top off water, which is about 25 times less than is present in saturated limewater. Consequently, this system alone is only good for tanks with very low calcium and alkalinity demand, though it can be used in conjunction with just about any other supplementation system (except limewater, which uses the same top off water).

If you use products like commercial play sand for this application, the cost can be very low. A 50-pound bag of calcium carbonate sand from Home Depot costs $3.50, or less than $0.008 per thousand meq of alkalinity. If you use hobby grade products, like Aragamight, the cost is more on the order of $12 per pound, or $2.64 per thousand meq of alkalinity. Tank salinity will not increase over time using calcium carbonate.

Another use of calcium carbonate is as the substrate in a reef tank. As organic molecules are degraded inside of the substrate, the pH can drop, and the calcium carbonate can dissolve just as it does inside of a CaCO3/CO2 reactor. This rate of dissolution ends up being slow, however, and typically cannot provide a tank with adequate amounts of calcium and alkalinity unless the demand is very low.

One-part balanced additive systems: Calcium Acetate

Calcium acetate is a product that has gotten relatively little publicity despite its apparent ease of use and the commercial availability to aquarists. In some ways it is similar to the combination of limewater and vinegar. When dissolved in water (fresh or salt), you have calcium ions and acetate ions. The acetate is rapidly metabolized by tank organisms to form bicarbonate, carbon dioxide, and water:

CH3COO (acetate) + 2 O2 → HCO3 + CO2 + H2O

This equation suggests that pH of such tanks may stay near the low end of normal, because of the excess carbon dioxide, but the practical experience of people using calcium acetate suggests that this is not a big concern.

Calcium acetate may also facilitate the conversion of nitrate to nitrogen gas (N2) in anoxic regions of live sand and rock by providing the carbon source necessary for the process (but this has not been demonstrated one way or the other). The equation below shows the process that could take place:

5 CH3COO (acetate) + 8 NO3 → 10 CO2 + 4 N2 + 13 OH + H2O

One of the sources of calcium acetate to aquarists is Salifert’s All in One (a product that also contains some strontium, amino acids, and some trace elements). It is a liquid product that can be poured directly into a tank with no immediate concerns about pH. The current version of their commercial product is 250,000-mg/L calcium acetate, so it contains the equivalent of 3,160 meq/L of alkalinity. This products sells in the US for about $31.50/L. Consequently, it costs about $10.00 per thousand meq/L of alkalinity. That price makes it very expensive for a tank with a large demand for calcium and alkalinity, but the zero initial costs make it attractive for small tanks, especially nano-reef tanks.

I have no information on the purity of the material, or the exact nature of the “trace elements” in it. Everything in the bottle will be delivered to the tank. It poses no unusual safety concerns. The upper limit to how much calcium and alkalinity can be supplied to a tank in this fashion depends on two factors. If the metabolism of acetate is rapid and the dose is very high, oxygen might be depleted. If the conversion is slow then acetate can build up in the tank (not itself a significant concern except perhaps at very high levels where it might confound an alkalinity test). Habib Sekha of Salifert has indicated that using the doses recommended on the bottle will not lead to either of these issues being problematic.

Overdosing is not expected to be an unusual problem, but if one makes significant additions in this fashion, the alkalinity will take time to show up completely in the tank because the acetate takes time to be metabolized. Consequently, I’d wait a day after adding it to measure alkalinity. Calcium measurement won’t be similarly impacted. Tank salinity will not increase over time using calcium acetate.

One-part balanced additive systems: Salt Mixtures

Another type of balanced one part additive is comprised of a simple dry mixture of sodium bicarbonate (or carbonate) and calcium chloride. Just as with the two-part additives described below, this type of system can be further formulated to have a natural seawater residue after removal of calcium carbonate. Tropic Marin’s Biocalcium seems to fall into this category, though it’s written descriptions are notoriously difficult to interpret. It costs about $8 for 500 grams (estimated to contain about 1800 meq of alkalinity), so that puts the cost at about $4.40 per thousand meq of alkalinity. It claims to add 79 trace elements to the tank, along with the calcium and alkalinity, but doesn’t specify amounts for any of them.

You cannot mix this type of additive in water prior to adding it to a tank. If you do, the calcium will react with the carbonate present to form insoluble calcium carbonate. Consequently, the directions advise adding it directly to the tank. If you do, be sure to add it in a high flow area away from corals (like a sump), as the solids are reported to irritate corals if they land on them.

If you use a product like this, be sure to keep it as dry as possible, even to the extent of keeping it in a sealed container to keep out atmospheric moisture. If moisture enters the mixture, it may allow the formation of undesirable calcium carbonate.

Continual use of products like this will increase the salinity in the tank. The rise in salinity over time can be roughly calculated, though not knowing exactly what is in it makes the calculation only a ballpark figure. For every 1000 meq of alkalinity added in this fashion these products will deliver on the order of 60 grams of other ions to the tank. In a tank with a low calcification demand (defined below to be 18.3 thousand meq of alkalinity per year in a 100-gallon tank (50 meq/day)) this effect will raise the salinity by 3 ppt per year (compared to a normal salinity of S =35). In a high demand tank (defined below to be 219 thousand meq of alkalinity per year in a 100-gallon tank (600 meq/day), the salinity will rise by 35 ppt in a year, or approximately doubling the salinity. Consequently, the salinity should be monitored closely in using this type of additive, especially in a tank with high calcification rates.

Two-part Balanced Additive Systems

There are now a plethora of two-part balanced systems for supplementing calcium and alkalinity. These are always liquid additives that you add equally to tanks to supplement both calcium and alkalinity. The rational is that the bicarbonate and carbonate that one might like to dose to supplement alkalinity are not readily compatible with the calcium that is also needed. So one portion contains calcium and the other contains the alkalinity.

In the simplest form, such a system would be provided by any calcium salt at one concentration in one bottle, and a carbonate alkalinity supplement at twice the equivalent concentration in the other bottle (twice because when calcium carbonate forms, it requires two units of alkalinity for each unit of calcium). Within that constraint, manufacturers have a fair amount of room to play.

Typically these additives claim go a step further. When the calcium and alkalinity are taken out of the picture, as they will be by calcification in the tank, then the ions that remain are often described as having the same ratios of ions as natural seawater. Assuming that this is true, then the “residue” is simply more salt for the tank water. Over long periods of time the salinity will build up due to this process (an effect that is quantified below), but there will be no significant buildup of specific ions in the tank.

In order to accomplish this, manufacturers could use a variety of calcium salts in the calcium portion, for example. They could use calcium chloride, calcium sulfate, calcium bromide, and a variety of other similar salts. They could also put magnesium and strontium in this portion as they would not be compatible with the alkalinity component.

The alkalinity portion of these systems is more complicated. As has been shown in other parts of this article, alkalinity can be provided as bicarbonate, carbonate, or hydroxide. I don’t know of any commercial supplements that use hydroxide, but the commercial ones do use bicarbonate, carbonate, and mixtures thereof. Consequently the pH varies substantially between brands, and the various brands of these products should not be thought of as identical for this reason, if no other. In order to attain the natural seawater residue, the alkalinity portion could contain sodium bicarbonate or carbonate, potassium bicarbonate or carbonate, lithium bicarbonate or carbonate, etc.

I’ve not seen any independent test of whether these actually produce a residue equivalent to natural seawater, but I’ve seen no particular reason to doubt it, at least for the major ions. When it comes to the trace elements that might concern some reef keepers, it seems unlikely that these products will be any less prone to having uncontrolled levels of trace compounds like copper than are commercial salt mixes, or any other supplement of calcium and alkalinity, but that remains to be determined (at least as far as I know).

One issue that has confused some reefkeepers, however, is the presence of trace elements. Assuming that these products are actually formulated with every ion such that a true natural seawater residue remained (lets call this the “ideal” product), then it will necessarily contain such ions as copper. Since it has been claimed that copper is elevated in reef tanks, and is toxic to many invertebrates, reef keepers have wrongly criticized this method as adding more copper. That’s actually not what would happen. Since these products leave a natural seawater residue, and since copper may be elevated in concentration in many reef tanks relative to seawater, then using these “ideal” products will actually LOWER copper levels because when the increase in salinity is corrected, the copper will drop.

For example:

  • You have copper in your tank at 4 ppb and salinity of S=35.
  • You add a two part additive that over the course of a month raises salinity to S=36, and raises copper to 4.02 ppb.
  • Then you correct the salinity back to S=35 by diluting everything in the tank with fresh water, and you get a final copper concentration of 3.9 ppb.

Does this happen in real products and not “ideal” products? I have no idea. But the statement by manufacturers that it contains all ions in natural ratios, including copper, should not be viewed as a concern that it is exacerbating a heavy metal problem.

The rise in salinity of these products over time can be very roughly calculated, though there are several reasons why this calculation is only an estimate. For every 1000 meq of alkalinity added in this fashion (and the matching amount of calcium) these products will deliver on the order of 60 grams of other ions to the tank. In a tank with a low calcification demand (defined later to be 18.3 thousand meq of alkalinity per year in a 100 gallon tank (50 meq/day)) this effect will raise the salinity by 3 ppt per year (compared to a normal salinity of S ~35). In a high demand tank (defined later to be 219 thousand meq of alkalinity per year in a 100 gallon tank (600 meq/day)), the salinity will rise by 35 ppt in a year, or approximately doubling the salinity. Consequently, the salinity should be monitored closely in using these types of additives, especially in a tank with high calcification rates.

The costs of these systems vary a bit. The original B-ionic costs about $34 for 1 gallon of both parts (10,600 meq of alkalinity), or about $3.20 per thousand meq of alkalinity. The bicarbonate version is substantially more expensive, at about $8.90 per thousand meq of alkalinity. They also vary in pH, as mentioned above. If your tank pH gets too high using one of them (such as the original B-ionic), then it is reasonable to switch to one that has a smaller pH raising effect (like the bicarbonate B-ionic).

Water Changes

The one thing going for water changes is that it is hard to screw them up chemically (aside from salinity, pH and temperature). The bad thing is that it is impossible to replace more than a tiny amount of lost calcium and alkalinity to a tank in this way. If salt mixes were available with higher than natural seawater levels of calcium and carbonate alkalinity, then this system could be a good one for tanks with a very low demand for calcium and alkalinity. Unfortunately, most salt mixes do not fit that description, and so the best that one can typically attain using this method, regardless of the number or size of the water changes, is not quite as good as the starting salt mix, which themselves are often not as good as natural seawater.

Cost Comparison

Each of the sections above has detailed a cost estimate of using that system, though in some cases there are a variety of different options to choose from that can significant impact cost. The table below is intended to be a rough guide to the initial and yearly cost of each of these systems for three types of 100-gallon tanks:

  1. Tanks with a light calcification load, defined as 50 meq of alkalinity per day (0.13 meq/L/day). This is the equivalent to the daily replacement of 0.3% of the tank volume with saturated limewater. This works out to 18,300 meq of alkalinity per year.
  2. Tanks with a medium calcification load, defined as 150 meq of alkalinity per day (0.4 meq/L/day). This is the equivalent to the daily replacement of 1% of the tank volume with saturated limewater. This works out to 55,000 meq of alkalinity per year.
  3. Tanks with a heavy calcification load, defined as 600 meq of alkalinity per day (1.6 meq/L/day). This is the equivalent of the daily replacement of 4% of the tank volume with saturated limewater. This works out to 219,000 meq of alkalinity per year.

Of course, smaller tanks will require less supplementation, and larger tanks will require more, and you can just scale the estimate to your tank based on it’s volume and your estimate of how much calcification is expected. Note also that some very high calcium and alkalinity demand tanks may be higher than the “high” demand tank.

As a general rule, a very small tank will probably be most economically served by a system with lowest set up costs (i.e., not a reactor of any kind), while for a larger amount of calcium and alkalinity, limewater and CaCO3/CO2 systems are likely to be the least expensive.

Table 1. Costs associated with various calcium and alkalinity supplementation schemes.
Yearly Cost for a 100 Gallon Tank ($)
MethodStart up Costs ($)Light LoadMedium LoadHeavy Load
Limewater(Bulk Lime)20-250+0.601.706.60(marginally possible)
Limewater(Aquarium Lime)20-250+20.8060.50241.00(marginally possible)
CaCO3/CO2 reactor350-6506.6019.3077.00
CaCO3w/o reactor0(not possible)(not possible)(not possible)
Two-part systems (original B-ionic)060.00180.00700.00
Two-part systems (bicarbonate B-ionic)0170.00490.001950.00
One-part Calcium acetate0190.00550.002,190.00
One-partSalt Mixtures083.00242.00960.00

Summary of Properties

Table 2 is a summary of the properties of the various schemes that have been discussed throughout the article. Depending on the nature of the reef tank itself, some of these attributes may be more or less important, and it is up to each individual aquarist to decide what best fits their needs. In every instance, the entries in this table represent my opinions about things that are described in more detail in the text. Other aquarists may disagree about assessments of how complex or risky something actually is, however.

Table 2. Summary of attributes associated with various calcium and alkalinity supplementation schemes.
MethodLimits to Addition Amount?Technical ComplexityDaily Work by Aquarist?Overdose RisksOther comments
Limewater(Aquarium Lime)Moderate, based on evaporationmediumYes(no with equipment)mediumLow metals

Reduced Phosphate

High pH

CaCO3/CO2 reactornonehighnolowDelivers metals and strontium / magnesium

Low pH

CaCO3 w/o reactorSevere limitslowYes(no with equipment)none
Two-part systems (original B-ionic)nonelowYes(no with equipment)Low to mediumSalinity Monitoring

Medium to High pH

Two-part systems (bicarbonate B-ionic)nonelowYes(no with equipment)lowSalinity Monitoring

Medium pH

One-part Calcium acetateNone to moderatelowYes(no with equipment)lowCarbon source

Medium pH

One-partSalt MixturesnonelowyeslowBottle/Moisture Stability?

Salinity Monitoring

Medium pH


Summary Guidelines for Beginners

Less experiences aquarists may have some difficulty in deciding which of these various attributes are most important for their situation. In this section, I provide some guidelines in selecting a balanced calcium and alkalinity supplement for certain types of tanks. Much of what is detailed below is opinion, and other aquarists may have different opinions.

Very Small Reef Tanks

A very small tank (say, less than 10-20 gallons, especially those without a sump) will likely be served best by a system that does not involve the expenses, complications, and space requirements that come with reactors. Unless the calcification demand is very high, the costs associated with any of the simpler additives (the two-part systems, Salifert’s All in One, Tropic Marin’s Biocalcium) will probably not be prohibitive, and their ease of use makes them prime candidates. Without a sump, Biocalcium may be harder to add without solids getting onto organisms, so either of the other two types may be a better choice. Simple drip limewater is also a less expensive possibility for these types of systems, but is best used when a sump is available.

Fish Only or Fish Only + Live Rock Tanks

These systems have smaller demands for calcium and alkalinity, though rapid coralline algae growth on live rock can itself provide a significant demand. Since the demands are lower than typical reef tanks, the size tank that is best served by the simpler additives described in the previous section is greater. Maybe up to 55-90 gallons for a tank with a fair amount of live rock, and even larger with small amounts of live rock.

Large Reef Tanks

A large tank (say, more than 100 gallons) will likely be served best by a system that can deliver calcium and alkalinity at a reasonable unit price. Limewater and calcium carbonate/carbon dioxide reactors are probably the best choices, with limewater not sufficing for the higher calcium demand tanks due to its limitation based on evaporation rates. If you are handy, you can put the system together yourself, especially limewater from a reservoir. If you aren’t handy, then by all means buy a complete system.

Medium Tanks

These are the tanks that most beginners have, containing 30-90 gallons. The logical choices to pick from are more numerous than for the systems described above, and will come down to a series of different factors.

  1. Does the tank have a sump where you can add high pH additives with less concern about nearby corals? If so, that’s a plus for limewater, Biocalcium, and the high pH two-part additives.
  2. Do you have a good skimmer or other source of aeration? If so, that is a plus for using limewater (that needs to suck CO2 from the air) or a calcium carbonate/carbon dioxide reactor (that needs to blow off excess CO2). If not, using Biocalcium, All in One, or a two-part additive that has a small pH effect (like the bicarbonate B-ionic) may be better choices.
  3. Is your home very tightly sealed, with possibly high indoor CO2? That is a plus for using limewater or a high ph two-part additive as they will counteract the tendency toward low pH.
  4. Do you have space under or behind the tank for equipment? If so, that is a plus for limewater or CaCO3/CO2 reactors that need space and are typically unattractive. If not, that is a big plus for the simpler additives (two-part systems, All in One, Biocalcium).
  5. Are you handy with complex systems? That is a plus for CaCO3/CO2 reactor systems and complicated auto top-off systems using limewater. If not, that is a plus for the simpler systems.
  6. Are you very concerned about copper or phosphate in your tank? If so, pick limewater.
  7. Are you going to leave the tank unattended for more than a few days? Pick a system with automatic delivery (many can be automated with the right selection of appropriate equipment, except Biocalcium).
  8. Will the tank have a very high demand for calcium and alkalinity? That is, will it have lots of fast growing corals? If so, a CaCO3/CO2 reactor would likely be the best choice.
  9. How much is cost an issue? For lowest cost, a cheap limewater drip will probably be best.

Of course there are many other issues to consider, and most of these were described in the body of the article. If you are just setting up a tank for the first time, I’d advise looking at existing tanks, and deciding what you want in it first. Then look to see what kinds of supplementation schemes these tanks use, and ask the owner how it is working out, and actually see for yourself what it looks like and what is involved. Then you’ll be in a good position to make an informed choice.

Happy Reefing!

References for Further Reading

  1. Calcium, by Randy Holmes-Farley:
  2. What is Alkalinity, by Randy Holmes-Farley:
  3. Calcium and Alkalinity, by Randy Holmes-Farley:
  4. More About Calcium and Alklainity, by Craig Bingman:
  5. Solving Calcium and Alklainity Problems, by Randy Holmes-Farley:
  6. The Chemical and Biochemical Mechanisms of Calcification, by Randy Holmes-Farley :
  7. Limits To Limewater…Revisited, by Craig Bingman:
  8. Solutions to pH problems, by Randy Holmes-Farley:
  9. Jaubert’s Method, the “Monaco System,” Defined and Refined, By Julian Sprung:
  10. The Relationship Between Alkalinity and pH, by Randy Holmes-Farley:
  11. It’s in the water, by Ronald Shimek:
  12. It is still in the water, by Ronald Shimek:
  13. Bingman, C. 1995. Precipitation of phosphate by limewater. Aquarium Frontiers Fall. and Bingman, C. 1996. Ion pairing, buffer perturbation and phosphate export in marine aquaria. Aquarium Frontiers 3(1):10-17
  14. Phosphate….What is it and why should you care, by Randy Holmes-Farley:–+Biochemistry+of+Aquaria&RecordNo=2481
  15. Phosphorus: Algae’s Best Friend, by Randy Holmes-Farley:
  16. Expanding the Limits of Limewater: Adding Organic Carbon Sources, by Craig Bingman:
  17. A Guide to Using Calcium Reactors, by Simon Huntington:
  18. Calcium Carbonate for CaCO3/CO2 Reactors: More Than Meets the Eye, by Craig Bingman:
  19. Alternative Calcium Reactor Substrates, by Greg Hiller:–+Biochemistry+of+Aquaria&RecordNo=1571
  20. Unpublished data from Simon Huntington
  21. Carbon Dioxide: Friend or Foe? by Randy Holmes-Farley:–+Biochemistry+of+Aquaria&RecordNo=2074
  22. Calcium Carbonate as a Supplement by Randy Holmes-Farley:


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