Calculating how much Calcium to add

This was prompted by a subscriber to our reefkeeping mailing list who wanted to know how much of a calcium supplement he needed to add to achieve a certain increase in the calcium level in his tank.

Another hobbyist replied, posting a suggested method given by Craig Bingman which follows below:

This is courtesy of Craig Bingman:

So you have a 25 gallon nominal system. Let's say that you actually have 20 gallons of water. That is about 76 liters of water.

You want to increase the calcium concentration from 320 to 400 mg/L, or an 80 mg/L increase. There are 40 grams per mole of calcium (ions.) 80 mg/L works out to a 2 millimolar increase in calcium concentration (40 mg/L = 1 mM.)

So you need 152 millimoles of calcium, or 0.152 moles.

There are at least three commercial forms of calcium chloride. One is anydrous, another has two waters per CaCl2, the third as six waters. The first two are most common, so I will run the math for them.,

Anhydrous CaCl2 FW 111.0

dihydrate FW 147.0

0.152 moles * 111.0 grams/mole = 16.9 grams CaCl2, anhydrous

0.152 moles * 147.0 grams/mole = 22.3 grams CaCl2, dihydrate.

I'm working on an outline for a book....... there is going to be a table for this sort of thing.