Aquarium Chemistry: Tap Water in Reef Aquaria

Many aquarists ask whether it is acceptable to use tap water for their reef aquaria. The answer obviously depends on what is in their tap water. We have all heard of aquarists who use only tap water and are seemingly very successful. We’ve also heard the opposite, such as folks that had significant problems until they purified their water. How is an aquarist to decide what to do?

Aside from the standard concerns about chlorine and chloramine,1 there are a variety of chemicals to be concerned with. One source for information on these chemicals is the annual water quality report from your municipal water supply, if on a public system. If you have a private well, you may need to get a test of your own water to know if it is suitable.

In this article, I will review some of the chemicals to be concerned with in tap water, and will show how 18 randomly-chosen public water supplies stack up. The chemicals to be concerned with generally fall into two groups. The first group is comprised of nutrients that may result in ongoing algae problems in the aquarium. These include phosphate, nitrate, and silica.

The second group is much broader, and includes those chemicals that are potentially toxic to reef aquarium inhabitants. Many of these, such as pesticide residues, are also toxic to humans, so elevated levels are unlikely to go uncorrected for long periods of time. Some, however, are more toxic to reef aquarium invertebrates than to humans. Copper falls into this category, for example. Consequently, the allowed levels of certain ions in drinking water can be higher than is acceptable in a reef aquarium.


As will become clear in this article, using tap water is risky business. Short of testing your own water, there is no way to be certain that your water is acceptable. Some testing with kits is likely desirable, but certain tests will require professional labs. The data supplied by the water company may tell you that the do not want to use it, but it cannot guarantee that yours is acceptable. Consequently, I generally recommend that folks not use tap water. However, if you choose to do so for cost or other reasons, I give some recommendations at the end of the article that are worth considering if you do use it.


Water Quality Reports

All public water supplies in the United States that supply more than 25 residents are required to supply customers with an annual water quality report. While there are specified guidelines on what should be in the report, in practice, the reports can include a wide variety of different types of information. Not all reports will cover all of the chemicals that aquarists are interested in. In fact, of the 18 that I surveyed, only two said anything about phosphate and only four said anything about silica. Often, water supplies compile monthly water quality reports that include much more data than the yearly report to customers. The October monthly report for my water supply includes data for more than 40 different inorganic compounds, even though only a few make it into the annual report. These monthly reports are not always available online, however.chem2.jpg

Confounding the interpretation of such reports is the fact that the potential impurities to be concerned about come from different sources. Nitrate typically comes from the source water (river, well, reservoir, etc). Silica and phosphate can come from the source, but can also be added to the tap water to raise pH and thereby reduce corrosion. Copper and lead typically come from customer pipes themselves, although my water company (MWRA) recently stated that they may add copper to a reservoir to reduce algae growth. In water quality reports, you need to be aware of what is being tested: a reservoir, water entering the supply pipes, or water leaving customer taps. Obviously, the last is what you really care about, but typically only copper and lead are tested this way. Other compounds are tested elsewhere in the system.

If you don’t have your annual water quality report handy, your best bet is to check their web site. This list of members of the Association of Metropolitan Water Agencies has links to many member web sites.


General Attributes of Tap Water

In addition to the “problem” issues with tap water, there are a variety of test results that aquarists might find useful if using tap water for a reef aquarium. Some water reports give data on calcium, magnesium, alkalinity, and pH, for example. Table 1 shows the results for these parameters in the 18 water supplies discussed in this article. Some do not divulge any of this information, and some only provide it in their monthly water reports. Nevertheless, it is apparent that these values vary considerably. At the bottom of the table, the same parameters are shown for saturated limewater (kalkwasser) for comparative purposes.

While the amount of calcium and alkalinity likely to come from evaporation replacement using any of these water supplies is not going to meet the demands of a typical reef aquarium, it may help. Some of the water supplies provide what could amount to more than 20% of the calcium and alkalinity needs for a reef aquarium with low to moderate demand.

The data for magnesium is also interesting, if sparse. The amount of magnesium added varies considerably even within a given water supply, from essentially none, to what might represent the daily demand for magnesium in an average reef aquarium (assuming magnesium is depleted at a rate about 2% of the calcium depletion rate).

pH also varies a lot. The pH of evaporation replacement water will be largely swamped by the pH of the aquarium, which is more strongly buffered. Nevertheless, if the pH of the tap water being used is 9.4, that value will be constantly tugging the aquarium to higher pH (which is likely beneficial for many aquarists), while using water with a pH of 6.2 might (or might not) be tugging it downward (depending on whether there is any real buffering at that pH).


Table 1. pH, calcium, and alkalinity in tap water (NR = not reported).
Water Supply (year of report) pH Calcium (ppm Ca2+) Alkalinity (meq/L)
Albuquerque (2002) 7.9 33 2.4
Boston (2003) data from October 2003 monthly report) 9.4 5 0.7
Cedar Rapids (2002) NR NR NR
Central Arkansas (2002) NR NR NR
Denver (2003) NR NR NR
Houston (2002) NR NR NR
Kansas City (2003) 7.2-7.6 67 – 93 4 – 5.5
Louisville, KY (2003) 7.9 47 (Mg2+ = 11) 1.5
Macon County, Georgia (2002) NR NR NR
Miami (2002) NR NR NR
Minneapolis (2002) NR NR NR
New York City (2002) 6.2 – 8.2 5 – 83 (Mg2+ = 1 – 35) 0 – 4.0
Orlando (2003) NR NR NR
Philadelphia (2002) NR NR 0.5 – 2.0
Phoenix (2002) 6.3 – 8.2 NR 2.8 – 4.4
San Francisco (2002) 7.4 – 9.8 4 – 31 (Mg2+ = 0 – 11) 0.3 – 2.4
San Diego (2002) NR NR NR
Sioux Falls, SD (2002) 8.0 – 9.2 34 – 83 0.3 – 2.1
Limewater 12.4 810 41


Chlorine And Chloramine In Tap Water

Before water is sent into supply pipes, it is first treated by the water company. Various treatments are performed to improve the quality of the water, and one of the most important treatments is to disinfect the water. Often this disinfection is accomplished with chlorine or chloramine. Since both chlorine and chloramine are toxic to many organisms, the residual disinfection chemicals present when the water arrives at your tap must be removed.

Chlorine can often be removed simply by letting the water age appropriately prior to use. Some aquarists aerate it, and others simply let it sit for a few days. Chlorine can also be removed with a variety of different commercial products that react with chlorine and reduce its toxicity.

Chloramine is not so readily removed by aging, and is more complicated to remove with commercial dechlorinating products since one must also remove the ammonia. These issues have been dealt with in detail in a previous article.1 The main point here is that if your water supply uses chloramine, you must be sure that you are treating the water in an appropriate fashion.

Both chloramine and chlorine are easily detected by commercial test kits. Regardless of the method that you use to remove them, testing to be sure it worked is good practice.


Nutrients In Tap Water: Nitrate

Aquarists with algae problems should consider reducing imports and increasing exports of nutrients such as nitrate,2 phosphate,3 and silica. 4 One way to reduce imports is to be sure that the water being used to replace evaporated water or to do water changes does not contain excessive nutrients. Most water quality reports have data on nitrate. Sometimes it is confounded with nitrite data, but rarely is there enough nitrite to be important.

Table 2 shows the nitrate levels for the 18 water supplies surveyed. The nitrate levels range from very low (Boston and Orlando, with less than 1 ppm nitrate as the maximum level) to very high (New York City, Orlando, and Miami, with more than 30 ppm nitrate maxima). Some aquaria may be able to process even the highest levels of nitrate in these tap waters without difficulty. But if excessive algae growth is a problem, then nitrate in tap water may be a contributing factor.

Table 2. Nitrate in tap water.
Water Supply (year of report) Nitrate level (ppm nitrate)
Albuquerque (2002) 0 – 2.3
Boston (MWRA; 2002) 0 – 0.6
Cedar Rapids (2002) 2 – 25
Central Arkansas (2002) 0.5 (only tested in 2001)
Denver (2003) 0.1 – 0.9
Houston (2002) 0 – 4
Kansas City (2003) 0.9
Louisville, KY (2003) Not reported
Macon County, Georgia (2002) 3 – 7.5
Miami (2002) 0 – 35
Minneapolis (2002) 1.7
New York City (2002) 0 – 43
Orlando (2003) 0 – 0.5
Philadelphia (2002) 2.5 – 22
Phoenix (2002) 0 – 34
San Francisco (2002) 0 – 2
San Diego (2002) Not reported
Sioux Falls, SD (2002) 10


Nutrients In Tap Water: Phosphate

Fewer water supplies provide information on phosphate, at least in their annual reports (Table 3). Some water supplies, like mine in Boston, supply such information in their monthly reports. Not all companies have such monthly reports on line, however. Some reports quote values for orthophosphate (which includes H3PO4, H2PO4, HPO42-, and PO43-) as well as total phosphorous (which includes orthophosphate and a variety of other forms). The latter is probably the important figure if they are different as it all likely becomes phosphate in the aquarium. With only a few data points to interpret, the data look to range from low to incredibly high at up to 5.4 ppm for New York. Using water with 5.4 ppm total phosphate would pose a serious risk for both algae problems, and inhibition of calcification of corals.

Table 3. Phosphate in tap water.
Water Supply (year of report) Phosphate (ppm)
Albuquerque (2002) Not reported
Boston (MWRA; 2003) Not reported in annual report(~0.01 ppm in 10/2003 monthly report)
Cedar Rapids (2002) Not reported
Central Arkansas (2002) 0.19 – 0.47
Denver (2003) Not reported
Houston (2002) Not reported
Kansas City (2003) Not reported
Louisville, KY (2003) Not reported
Macon County, Georgia (2002) Not reported
Miami (2002) Not reported
Minneapolis (2002) Not reported
New York City (2002) 0 – 5.4
Orlando (2003) Not reported
Philadelphia (2002) not reported
Phoenix (2002) Not reported
San Francisco (2002) Not reported
San Diego (2002) Not reported
Sioux Falls, SD (2002) Not reported


Nutrients In Tap Water: Silica

When I first started a reef aquarium, it was the very high levels of silica in my tap water that lead me to purchase an reverse osmosis/deionizing system (RO/DI) to purify it. In retrospect, that reason may have been faulty, as I now add silica4 to my aquarium in about the same amount as if I were using tap water to replace evaporated water. Nevertheless, I am also glad that I use RO/DI purification for the concerns about metals that are discussed later in this article.


However, many aquaria, especially new aquaria, can suffer from problems with diatoms. In such a system, silica in tap water can significantly contribute to the problem, with tap water having as much as 80 ppm silica in it (in Albuquerque; Table 4). At that level, even mature aquaria may have significant difficulties dealing with the silica additions from evaporation replacement and water changes.

Table 4. Silica in tap water.
Water Supply (year of report) Silica (ppm)
Albuquerque (2002) 35 – 80
Boston (MWRA; 2003) Not reported in annual report(2 – 3 ppm in 10/2003 monthly report)
Cedar Rapids (2002) Not reported
Central Arkansas (2002) < 1
Denver (2003) Not reported
Houston (2002) Not reported
Kansas City (2003) Not reported
Louisville, KY (2003) Not reported
Macon County, Georgia (2002) Not reported
Miami (2002) Not reported
Minneapolis (2002) Not reported
New York City (2002) 0.8 – 24.4
Orlando (2003) Not reported
Philadelphia (2002) Not reported
Phoenix (2002) Not reported
San Francisco (2002) 5 – 6
San Diego (2002) Not reported
Sioux Falls, SD (2002) Not reported


Toxic Metals In Tap Water: Copper

One of the biggest concerns for aquarists considering the use of tap water is copper. Copper is more toxic to marine invertebrates than it is to humans, so the EPA allows more in tap water than an aquarist would want in an aquarium. Specifically, the EPA has an action level at 1.3 ppm copper in tap water, and it is only a big problem (to the EPA) if more than 10% of the homes on the water supply exceed that level. For that reason, water supplies typically report the 90% level, meaning that 10% of homes exceed that stated value. Many also report the maximum level in any home tested.

Table 5 shows copper levels for a small selection of homes for all of the cities in this study. The reason that copper must be tested at the home tap is that most of the copper in public water supplies comes from the pipes in the home itself. Further, newer does not necessarily mean better, as fresh copper pipes might be even more inclined to release copper to the water than those that have had decades to build up protective coatings.

How much copper is too much? That is difficult to say, and certainly varies from organism to organism. In a recent test,5 Ron Shimek added copper to natural seawater and looked at the effect on sea urchin larvae. He found that concentrations above 10 ppb decreased the larval survival after 48 h, and that concentrations above 100 ppb killed all of them. Whether that translates into particular copper levels to be concerned about in typical reef aquaria is open to discussion. Nevertheless, it points out that copper is potentially toxic at levels well below the EPA action level of 1.3 ppm. That hypothesis is well supported by the literature on copper toxicity.6 Canadian Water Quality Guidelines for the Protection of Aquatic Life
suggest that copper be kept to less than a few ppb in fresh water, but they do not provide a salt water recommendation.

So how do the water supplies stack up with respect to copper? For some homes, not well. Ten percent of the homes tested in Miami have more than 1100 ppb copper in their water. In Kansas City, Orlando, Phoenix, and certain districts in Houston, ten percent of the homes have more than 500 ppb copper. At least one of my neighbors in Boston has 1100 ppb copper.

What’s the best case? That’s hard to say for sure, but perhaps Central Arkansas, where ninety percent of homes have copper below 50 ppb. Nevertheless, ten percent of homes are above that level, and some may be significantly above that level.

Based on the data, if your home is among the highest in your district, your water likely will be unacceptable. Since most aquarists do not know the copper levels in their water, it can be a risk to use it. If you do choose to use it, there are some simple actions that you can take to help:

  1. Test the water for copper with a test kit. Some of the kits available to hobbyists (Seachem and Hach LP Cube, #21938-00), for example) claim to detect copper at fairly low levels (10 ppb for Seachem and 50 ppb for Hach). Whether those limits are attained in practice or not, I do not know. But they likely will easily detect the very high levels in some tap water (500-1000+ ppb copper). Thus they can at least be used to screen out the worst offenders.
  2. Let the water run for 5-10 minutes before collecting it. This will greatly reduce the likelihood of getting a high copper (or lead) level from water that has been sitting in your pipes for an extended period of time.
Table 5. Copper in tap water (tested at homes).
Water Supply (year of report) Copper Level (ppb), 10% of homes above this level Maximum Copper level (ppb)
Albuquerque (2002) 200 <1300
Boston (MWRA; 2003) 120 1100
Cedar Rapids (2002) 100 <1300
Central Arkansas (2002) 50 <1300
Denver (2003) 190 <1300
Houston (2002) 50 – 546 (depends on district) <1300
Kansas City (2003) 690 >1300
Louisville, KY (2003) 230 Not reported
Macon County, Georgia (2002) 110 <1300
Miami (2002) 1100 >1300
Minneapolis (2002) 300 <1300
New York City (2002) 310 430
Orlando (2003) 590 Not reported
Philadelphia (2002) 300 <1300
Phoenix (2002) 540 >1300
San Francisco (2002) 120 350
San Diego (2002) 346 <1300
Sioux Falls, SD (2002) 89 <1300


Toxic Metals In Tap Water: Lead And Chromium

Water supplies are also required to measure lead levels. Since lead has become a significant concern with respect to development of children exposed to it, the levels of lead in water supplies have been greatly reduced in recent years. Still, some homes and water supplies have lead pipes and solder, and those concerns have been the source of many new regulations by the EPA (including raising the pH of tap water to reduce corrosion).


Table 6 shows data for lead tested at a small sample of homes in each district. The values obviously vary greatly, from less than 1 ppb maximum in Albuquerque to more than 50 ppb maximum in Boston and San Francisco.

Whether these lead levels are a concern or not isn’t entirely clear. There are undesirable effects noted to organisms6 in the tens of ppb lead. Canadian Water Quality Guidelines for the Protection of Aquatic Life suggest that lead be kept to less than a few ppb in fresh water, but they do not provide a salt water recommendation. It also bioaccumulates in organisms, so constant low doses can build up a significant body burden. However, the exact effects in aquaria are not known.

Table 6. Lead in tap water (tested at homes).
Water Supply (year of report) Lead Level, 10% of homes above this level Maximum Lead level (ppb)
Albuquerque (2002) 0 0
Boston (MWRA; 2003) 11 62
Cedar Rapids (2002) 4 >15
Central Arkansas (2002) 1 > 4
Denver (2003) 9 >15
Houston (2002) 2 – 5 (depends on district) >15
Kansas City (2003) 15 >15
Louisville, KY (2003) 4.5 Not reported
Macon County, Georgia (2002) 14 >15
Miami (2002) 5 >15
Minneapolis (2002) 5.8 >15
New York City (2002) 15 38
Orlando (2003) 2 Not reported
Philadelphia (2002) 13 >15
San Francisco (2002) 7 59
San Diego (2002) 0 <15
Sioux Falls, SD (2002) 0.7 <15

Some water supplies also give values for chromium testing (Table 7). Like lead and copper, chromium can be toxic to marine organisms. Canadian water quality guidelines suggest less than 56 ppb of chromium (III) compounds and less than 1.5 ppb for chromium (IV) compounds. What portion of chromium ends up in different forms in aquaria is not known, but chromium may exceed the suggested guidelines in some water supplies.

Table 7. Chromium in tap water.
Water Supply (year of report) Chromium (ppb)
Albuquerque (2002) 0 – 22
Boston (MWRA; 2003) Not reported in annual report (<0.6 ppb in 10/2003 monthly report)
Cedar Rapids (2002) Not reported
Central Arkansas (2002) Not reported
Denver (2003) Not reported
Houston (2002) Not reported
Kansas City (2003) 1.1 – 1.7
Louisville, KY (2003) 2.3
Macon County, Georgia (2002) Not reported
Miami (2002) 0 – 0.2
New York City (2002) Not reported
Orlando (2003) Not reported
Philadelphia (2002) Not reported
Phoenix (2002) 0 – 76
San Francisco (2002) Not reported
San Diego (2002) Not reported
Sioux Falls, SD (2002) Not reported



Based on data such as that shown above, there are many reasons to be concerned about tap water for use in reef aquaria. In some cases, especially the nutrients, one can learn quite a bit about the suitability of your tap water from a water quality report. Unfortunately, some of the potential problems with tap water may come from your own pipes, and so may not be reflected in the report. My recommendation is to be on the safe side and purify the tap water (via reverse osmosis, deionizing resins, or any of a variety of other related methods).

If you decide to use tap water because of the cost of purification, here are some recommendations:

  1. Be sure to deal appropriately with chlorine or chloramine in the water.
  2. Let the water run for 5-10 minutes before collecting it. This will greatly reduce the likelihood of getting a high copper or lead level from water that has been sitting in your pipes for an extended period of time.
  3. You should consider getting a copper test kit to reduce the concern that your tap water has very high levels of copper. Seachem and Hach claim fairly low detection limits for copper. They likely won’t distinguish the lowest levels, or guarantee that it is low enough to be ideal, but they should highlight the exceedingly high levels that some homeowners have.
  4. If you have an algae problem, test the tap water for nitrate, phosphate, and silicate (the latter in the case of diatoms).

Happy Reefing!



  1. Chloramine and the Reef Aquarium by Randy Holmes-Farley Reefkeeping, November 2003.
  2. Nitrate in the Reef Aquarium by Randy Holmes-Farley Advanced Aquarist, August 2003
  3. Phosphorus: Algae’s Best Friend by Randy Holmes-Farley Advanced Aquarist, September 2002
  4. Silica in Reef Aquaria by Randy Holmes-Farley Advanced Aquarist, January 2003
  5. The Toxicity of Some Freshly Mixed Artificial Seawater; A bad Beginning For a Reef Aquarium by Ronald Shimek Reefkeeping, March 2003.
  6. PAN Pesticides Database – Chemical Toxicity Studies on Aquatic Organisms
  7. PAN Pesticides Database – Chemical Toxicity Studies on Aquatic Organisms
  Advanced Aquarist, Advanced Aquarist

 Randy Holmes-Farley

  (29 articles)

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