The amount of CaCl2 you will need to add depends upon the hydration state of the compound. CaCl2 comes anhydrous(rarely), dihydrous (very common), monohydrous, or hexahydrous.
To find out your proper dosing, it depends upon who you ask. The PPS folks (like myself) will tell you to keep 20 to 30 ppm of Ca. Get a Ca test kit and find out your current concentration. Like the PPS recommends, I'd double your sample volume to get a more accurate reading. Once you know your concentration, you can dose enough to maintain correct levels.
Also, if you don't have one already, pick up a GH test kit. With the GH and Ca test kits, you can calculate your magnesium easily.
-Dustin |