Reef Supplementation Part One

by John Tullock

Every reef aquarist must have a method for replenishing the calcium carbonate depleted from the aquarium as corals and some other invertebrates grow and add mass to their skeletons. Over the years, several methods have been advocated for accomplishing this. Calcium supplements, mostly solutions of calcium chloride, were among the earliest reef products to appear on the market. Next came the use of Kalkwasser, or calcium hydroxide solution, and soon thereafter we saw the development of calcium reactors using various forms of calcium carbonate.

As highlighted in an article in the most recent issue of CORAL, the “Balling method” has become popular in the United States after enjoying several years of popularity in Europe. Like the other methods of ion replenishment, this technique has some drawbacks. There are also skeptics who doubt the method’s basic assumption, i.e., that steps must be taken to maintain the aquarium’s “ionic balance.”

Dealers have a large selection of supplement products to meet the needs of reef hobbyists, each with its own claims of efficacy, purity, concentration, etc. Some of the manufacturers of supplements support brick-and-mortar retailers, and many do not. How, therefore, do you choose among the numerous options out there?

The first step is understanding the chemistry involved. Seawater is a complex mixture of ions, and the chemistry of their interactions can be complicated. However, to evaluate manufacturer’s product claims, you do not need to understand the chemistry of seawater, only the chemistry of supplements. To do that requires defining some terms and doing a little math. In this post and future ones, I will walk you through the process.

Manufacturers provide information on their product labels that in some cases allows you to figure out what is actually in the bottle, but they often do so in ways intended to make those calculations difficult. For example, consider this information from the Thrive Aquatics catalog:

“Thrive’s Calcium is formulated at 196,821 mg/kg, which is a saturated solution of calcium chloride.”

How does this translate into practical terms? First, mg/kg is the same as parts per million (assuming the solvent is pure water). It is easier to work with smaller numbers, so let’s divide the mg by 1000 to get grams per liter. Thus, the product contains 197 grams per liter. Of what? Calcium, or calcium chloride? How do we answer that question? Turning to Wikipedia, we find that the molecular weight of anhydrous calcium chloride is 110.98 grams per mole. (A mole is the molecular weight expressed in grams. The molecular weight of CaCl2 is determined by adding up the atomic weights of calcium (one atom per molecule) and chlorine (2 atoms per molecule). Calcium is 40 and chlorine is 35.5 (rounding off for simplicity here). Adding these up gives 111 grams. Therefore, calcium chloride is 40/111 = 36% calcium.

To get 197 gm/L (the 196, 821 mg/kg stated in the catalog) you would need 197/.36 = 547 grams per liter of calcium chloride. This is not, however, equivalent to a saturated solution, because the solubility of calcium chloride in water is 745 grams/L. That would work out to 745*.36 = 268.2 gm/L or 268,200 mg/kg of calcium ions. Therefore, either the solution is not saturated, or the amount of calcium it contains is stated incorrectly.

Nevertheless, the claims made by Thrive are consistent with the notion that theirs is the most highly concentrated supplement among several competitors.

Over the coming days, I will be sharing more calculations of this type. In the meantime, please email me and let me know what methods and supplements you recommend. If you have time, tell me why.

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