Last month I reported on the purity of several brands of calcium chloride, including the relatively inexpensive Dowflake 77-80% Calcium Chloride available at Home Depot for about $12 for 50 pounds. This month, I will show how to use that material as part of an inexpensive homemade two-part calcium and alkalinity supplement. The only materials required are calcium chloride, baking soda (from a grocery store), and Epsom Salts (which is inexpensive and available at most drug stores).

This additive system will be balanced in that equal addition of the two parts will provide calcium and alkalinity in approximately the same ratio as used in calcification by corals and coralline algae. In that sense, it is the same as the commercial two-part additives. One part is the calcium chloride dissolved in water, and the second part is baking soda (either baked prior to use, or not) dissolved in water. This balance is very important in that an aquarium such a balanced additive system is unlikely to undergo big swings in calcium and alkalinity, as can happen if an aquarist using independent additives were to inadvertently overdose one relative to the other. This problem is surprisingly common, and was the reason that I wrote an article on how to solve calcium and alkalinity problems, and why I only described balanced additive systems of
various types in my article on how to select a calcium and alkalinity additive system.

A “third” part of this additive system represents the Epsom Salts (magnesium sulfate heptahydrate) dissolved in water, and is only required once in a while (perhaps added once every 1-2 months). It serves to prevent sodium and chloride from rising significantly relative to the other major ions, most notably magnesium and sulfate. You can buy Epsom Salts from most drug stores, but if you cannot find it, you can buy it online for $2.69 for 64 ounces. Epsom Salts sold in drug stores is generally labeled USP, which stands for United States Pharmacopoeia, and is an assurance that it is suitable for consumption by people.

The primary ions in seawater, in decreasing order of concentration, are chloride, sodium, sulfate, magnesium, calcium, and potassium. Using these recipes will prevent chloride (from the calcium chloride) and sodium (from the baking soda) from rising relative to the magnesium and sulfate. Preventing magnesium depletion is especially important in maintaining appropriate calcium and alkalinity in aquaria, so this third part of the additive system can be important.

This solution is not perfect. If commercial two-part additives are formulated as they claim to be with all of the remaining ions present in seawater ratios, then this recipe is not as good as those formulations. Whether the commercial additives are formulated correctly or not, I cannot say. The advantages of the recipes in this article are primarily cost. A drawback to nearly all commercial two-part additive systems is that they are quite expensive for all but the smallest aquaria. These recipes solve that cost problem. Using these recipes, however, sulfate will rise slightly over time, and certain other ions, such as bromide and fluoride, may decline over time. But it is a better recipe than calcium chloride and baking soda (or commercial “buffers”) alone, and I believe that it is adequate for aquarists for whom cost is a significant factor.

Contrasting The Recipes

I actually provide two recipes in this article.

Recipe #1 is for use in aquaria where the pH is normal to low. It will have a pH raising effect due to the elevated pH of the alkalinity part, as do most of the commercial two-part additives. The rise that you get will depend on the alkalinity in your aquarium, and, of course, on how much you add. If you add on the order of 0.5 meq/l of alkalinity then the pH will rise about 0.15 to 0.35 pH units immediately upon addition (and higher locally before it has a chance to mix into the whole aquarium).

So if you are using limewater (kalkwasser) and the aquarium runs at pH 8.4 or above, this recipe is not the best choice. Otherwise, it is likely to be a good option.

Recipe #2 is for use in aquaria where the pH is on the high side (above 8.3 or so). It will have a very small pH lowering effect when initially added. The drop that you get will depend on the alkalinity in your aquarium, and, of course, on how much you add. If you add on the order of 0.5 meq/l of alkalinity then the pH will drop by about 0.04 pH units immediately upon addition. The pH may later rise if the aquarium is permitted to blow off excess CO₂. This recipe is half as concentrated as Recipe #1.

Figure 1. Corals like this Montipora capricornis in the aquarium of Marc Levenson use a lot of calcium and alkalinity as they grow.

So if you are using limewater (kalkwasser) and the aquarium runs at pH 8.4 or above, this recipe may be the best choice.

Recipe #1

In this recipe you make 3 stock solutions. Two are used frequently, and one is only used occasionally to balance things out.

You can mix and store the solutions in any all-plastic or glass container. Plastic 1-gallon milk cartons (typically made of HDPE, high density polyethylene) can be a good choice.

Part 1: The Calcium Part

Dissolve 500 grams (about 2 1/2 cups) of calcium chloride dihydrate (Such as Dowflake 77-80% calcium chloride) in enough water to make 1 gallon of total volume. You can dissolve it in about 1/2 gallon of water, and then pour that into the 1 gallon container and fill to the top with more fresh water. This solution is then about 37,000 ppm in calcium.

If you use an anhydrous calcium chloride (such as Kent’s Turbo Calcium or Peladow Calcium Chloride (a brand sold by Dow that some believe may just be a dehydrated equivalent of the Dowflake), then you should use about 20% (1/5) less solid calcium chloride to make the recipe. Note that the solution will get quite hot when dissolved anhydrous calcium chloride.

Part 2: The Alkalinity Part

Spread baking soda (594 grams or about 2 1/4 cups) on a baking tray and heat in an ordinary oven at 300 °F for 1 hour to drive off water and carbon dioxide. Dissolve the residual solid in enough water to make 1 gallon total. This dissolution may require a fair amount of mixing. Warming it speeds the dissolution process. This solution will contain about 1,900 meq/L of alkalinity (5,300 dKH). I prefer to use baked baking soda rather than washing soda in this recipe as baking soda from a grocery store is always food grade, while washing soda may not have the same purity requirements. Arm & Hammer brand is a fine choice.

These two solutions are added as frequently as necessary to maintain calcium and alkalinity. In some light to medium demand aquaria, you may not need daily additions, especially for calcium. Nevertheless, unless testing shows that you should do otherwise, add equal amounts of Parts 1 and 2 over the course of a week.

Part 3: The Magnesium Sulfate

Dissolve a 64 ounce container of Epsom salts (about 8 cups) in enough purified fresh water to make 1 gallon total volume. This solution is added much less frequently than the other two parts. Each time you finish adding a gallon of both parts of the Recipe #1, add 610 mL (2 1/2 cups) of this stock solution. You can add it all at once or over time as you choose, depending on the aquarium size and set up. Add it to a high flow area, preferably in a sump. In a very small aquarium, or one without a sump, I’d suggest adding it slowly; especially the first time you do so to make sure that corals don’t get blasted with locally high concentrations of magnesium, sulfate, or any impurities in your Epsom Salts. The first time that you add it, you might add a small portion and make sure there isn’t any problem before proceeding to add the remainder. This solution contains about 47,000 ppm magnesium and 187,000 ppm sulfate.

Recipe #2

In this recipe you make 3 stock solutions. Two are used frequently, and one is only used occasionally to balance things out.

You can mix and store the solutions in any all plastic or glass container. Plastic 1-gallon milk cartons (typically made of HDPE, high density polyethylene) can be a good choice.

Part 1: The Calcium Part

Dissolve 250 grams (about 1 1/4 cups) of calcium chloride dihydrate (Such as Dowflake 77-80% calcium chloride) in enough water to make 1 gallon of total volume. You can dissolve it in about 1/2 gallon of water, and then pour that into the 1 gallon container and fill to the top with more fresh water. This solution is then about 18,500 ppm in calcium.

If you use an anhydrous calcium chloride (such as Kent’s Turbo Calcium or Peladow Calcium Chloride (a brand sold by Dow that some believe may just be a dehydrated equivalent of the Dowflake), then you should use about 20% (1/5) less solid calcium chloride to make the recipe. Note that the solution will get quite hot when dissolved anhydrous calcium chloride.

Part 2: The Alkalinity Part

Dissolve 297 grams of baking soda (about 1 1/8 cups) in enough water to make 1 gallon total. This dissolution may require a fair amount of mixing. Warming it speeds the dissolution process. This solution will contain about 950 meq/L of alkalinity (2660 dKH). Arm & Hammer brand is a fine choice of baking soda brand.

These two solutions are added as frequently as necessary to maintain calcium and alkalinity. In some light to medium demand aquaria, you may not need daily additions, especially for calcium. Nevertheless, unless testing shows that you should do otherwise, add equal amounts of Parts 1 and 2 over the course of a week.

Part 3: The Magnesium Sulfate

Dissolve a 64 ounce container of Epsom salts (about 8 cups) in enough purified fresh water to make 1 gallon total volume. This solution is added much less frequently than the other two parts. Each time you finish adding a gallon of both parts of the Recipe #1, add 305 mL (1 1/4 cups) of this stock solution. You can add it all at once or over time as you choose, depending on the aquarium size and set up. Add it to a high flow area, preferably in a sump. In a very small aquarium, or one without a sump, I’d suggest adding it slowly; especially the first time you do so to make sure that corals don’t get blasted with locally high concentrations of magnesium, sulfate, or any impurities in your Epsom Salts. The first time that you add it, you might add a small portion and make sure there isn’t any problem before proceeding to add the remainder. This solution contains about 47,000 ppm magnesium and 187,000 ppm sulfate.

Calculation Rational For The Recipes

The calculation rational is shown below for recipe 1. The rational for recipe 2 is the same, except everything is divided by 2 and there is no baking of the baking soda. This section is provided for those who want to know how the recipe is devised, are concerned that there might be an error, or who might want to change it slightly. It is not necessary to read the following section if all that you want to do it use it.

The Dowflake material is supposed to contain 77-80% calcium chloride. From the Dow Flake web site, it has a bulk density of 0.82 – 0.96 g/dry mL, or 194 – 227 grams/level measuring cup. We will assume that it is 78.5% calcium chloride by weight and weighs 200 grams per level measuring cup. Since calcium comprises 36% of calcium chloride, by weight, each cup contains 200 x 0.785 x 0.36 = 56.5 grams of calcium.

Consequently, dissolving 2 1/2 cups (500 g) of Dowflake per gallon = 141 grams of calcium per gallon, or 37,300 mg/L. You final concentration will vary with how much moisture was actually in the calcium chloride, and how well it packed in your measuring cup. A concentration of 37,300 ppm calcium is equivalent to 0.93 molar.

When calcification takes place, there are 2 moles of alkalinity lost for every 1 mole of calcium. So we need to match the calcium above with 1.86 molar baking soda (sodium bicarbonate) equivalents (before or after baking, the baking doesn’t change the alkalinity). As I measure it, Arm & Hammer baking soda weighs about 264 grams per level measuring cup. Since sodium bicarbonate has a molecular weight of 84 g/mole, we need to dissolve 1.86 x 84 = 156 grams/L, or about 594 grams (2 1/4 level measuring cups) of baking soda per gallon. Note that it doesn’t matter how many grams the 594 grams of baking soda turns into on baking. All you are doing is changing the amount of carbon dioxide in the baking soda:

2 NaHCO₃ → Na₂CO₃ + H₂O + CO₂

More or less baking will only alter the pH rise on addition to the aquarium. However, substantial under baking may make it impossible to attain full dissolution of the solid material in the recipe as sodium bicarbonate is less soluble than sodium carbonate (and is why Recipe#2 is more dilute).

When you add 1 gallon of each of these additives, there will be a residue of ions remaining after calcification. These are mostly sodium and chloride, and the amounts of those two added are equal in numbers (e.g., moles), but slightly different in weight-based concentrations like ppm since they do not weigh the same.

After adding 594 grams of baking soda (1 gallon of Recipe #1), we will have added 163 grams of sodium. In natural seawater, magnesium is present at about 12.0% of the sodium concentration (by weight). In order to match the magnesium additions to the sodium additions, and leave them in a natural ratio, we then need to add 12% of 163 grams, or 19.5 grams of magnesium for every gallon of the two-part additive that we add.

After adding 500 grams of Dowflake Calcium Chloride (1 gallon of Recipe #1), we will have added 250 grams of chloride. In natural seawater, magnesium is present at about 6.7% of the chloride concentration (by weight). In order to match the magnesium additions to the chloride additions, and leave them in a natural ratio, we then need to add 6.7% of 250 grams, or 16.5 grams of magnesium for every gallon of the two-part additive that we add.

Additionally, we may want to account for magnesium that is actually incorporated into the coral skeletons. For this calculation, I have assumed that the amount of magnesium incorporated is about 6.5% of the calcium level (by weight), or about 2.5% of the skeleton by weight. In the course of adding this gallon of both parts of the two part supplement, we added 141 grams of calcium, so we need to add 0.065 x 141 = 9 grams of magnesium to account for this deposition.

So we have to decide, do we want to match magnesium to the sodium, or to the chloride, or to an average of them, or something else? To be sure to prevent magnesium depletion, I selected the most conservative option, and added enough magnesium so that it is not depleted relative to sodium, and then is slightly raised relative to chloride. You can, of course, add more or less of Part 3 as you choose, or as you determine is needed based on magnesium measurements.

Since our magnesium supplement is 47,000 mg/L in magnesium, we need to add (9 +19.5) grams / 47 g/L = 610 ml of the magnesium solution.

If we wanted to balance sulfate (against sodium) rather than magnesium (against sodium), we would actually add less of the magnesium sulfate solution. In that case, using the same math suggest that we would add 220 mL instead of 610 mL (to balance sulfate against sodium. Since I am less concerned with elevated sulfate than I am with depleted magnesium, I have elected to use the full dose that maintains magnesium levels. However, if you choose to add magnesium in other ways, or are more concerned about sulfate than magnesium, using a smaller addition of the magnesium sulfate solution is an easy option.

Interestingly, the potassium present as an impurity in the Dowflake works to our advantage. It is present at 3600 ppm in the solutions that I used for testing in the previous article. In those same solutions, the calcium level is about 100,000 ppm, and chloride is about 177,000 ppm. So the residue has 3600 ppm of calcium for every 177,000 ppm of chloride for a ratio of K⁺/Cl^– of 0.0203 by weight. Seawater has 400 ppm of potassium and 19,350 ppm of chloride, for a ratio of K⁺/Cl^– of 0.0207 by weight. So the formula is quite well balanced in potassium (assuming that there is not a lot of excess potassium in the baking soda or in the Epsom Salts).

Residue Remaining From Recipe #1

After 1 year of adding 8 ppm of calcium and the accompanying 0.4 meq/L (1.1 dKH) of alkalinity per day (41 mL of both parts per day or 4 gallons of both parts per year in a 50 gallon aquarium; including the effect of the magnesium sulfate solution, 2440 mL/year), the following residue would remain after calcification and adjustment for salinity (there is roughly a 29% rise in salinity over a year using this addition rate without water changes):

Note that if an inexpensive source of magnesium chloride of suitable quality were found, these recipes could be improved so that sulfate did not rise at all. Unfortunately, I have not found such a source.

Summary

This inexpensive two-part additive system should be useful for aquarists who want to use a two-part additive, but have been deterred by the cost of commercial systems.

If you are using it, be sure to check the calcium and alkalinity values over time, even after you establish a routine that looks to do the job. Because of the uncertainty in the amount of moisture in these products when purchased, and in the amounts that you actually measure out, the system may not be perfectly balanced, and a slow drift toward elevated calcium or elevated alkalinity may take place.

Measuring magnesium once in a while is also likely a good move, just in case it is being used more or less rapidly than expected.

Good Luck and Happy Reefing