I'm continuing Javalee's discussion started in "My newly replanted 10g". He last wrote the following :
"I bought 2 oz. CaCl2 on a brewery website for cheap! I discovered these home-brewery sites are great places to get mineral supplements for plants. They even sell something called Burton's Water Salts that is designed to raise the GH of water used for brewing to the standards of some river water in England that is famous for making good ale. It adds Ca, Mg, and K in the correct proportions and it's cheap if anyone needs to raise GH. The only reason I didn't use it is because using soil packets left me with some spots that I fear could turn anaerobic and the BWS has sulfate in it. I got Epsom salts for Mg. I just dissolved and added them in as close to a 4:1 Ca:Mg ratio as I could."
CaCl2 is probably the best chemical to bring GH up without increasing pH. In my book, I advocated shells, oyster grit, mineral pills, etc. These are all mostly CaCO3 (calcium carbonate). I think they'll work, but they will probably increase the pH.
I highly recommend CaCl2 for calcium dosing. Several lab people that I work with used CaCl2 (upon my recommendations) to bring the GH up in their softwater tanks. Invariably, it stimulated plant growth. I did not recommend CaCl2 in my book, because I didn't think that non-laboratory folks had access to CaClw. However, Javalee seems to have found a good CaCl2 source. I trust beer brewers!
Chloride has no potential for toxicity while nitrate and sulfate salts do. Soil bacteria will convert nitrate to nitrite and sulfate to hydrogen sulfide. Both products are toxic. The amount of chloride you would add to bring GH up doesn't come close to inhibiting plant growth.
Other chemicals like magnesium, iron, etc can be added as sulfate salts, because they are required by plants in much smaller quantities than calcium. (
For folks with soft water tanks, I would go with Javalee's prescription for bringing GH up in softwater tanks:
Add CaCl2 and MgSO4 (Epsom Salts) in a 4:1 ratio (approximately).
, but since I don't, I just assumed that Ca+2 and Mg+2, having the same charge, would react in the same way with the reagent in my GH test kit. So I raised the degree of hardness first with Mg, then 4 times that amount with Ca. I may be off on this, but I didn't think my plants would punish me too much if I were. 
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