The difference is water in the crystals, not more solid impurities. It just happened that Dowflake is what my Home Depot sold when I bought some for impurity testing.
This article details the impurity analysis:
Purity of Calcium Chloride
http://www.advancedaquarist.com/issues/mar2004/chem.htm
Here's the section from my upcoming article on an improved DIY two part additive discussing substitutes:
Substitutes for Dowflake Calcium Chloride
If Dowflake calcium chloride or a repackaged version (such as All-Clear) cannot be located, one may be able to substitute Peladow or Dow Mini-Pellets which are dehydrated versions of Dowflake (that is, they have less water in the crystals). In addition to the Peladow brand name, Peladow is also sold as Prestone Driveway Heat and possibly other common brands. Kent Turbo Calcium is also suitable and is an anhydrous calcium chloride. Any FCC (food), USP (pharmaceutical) or BP (pharmaceutical) grades of calcium chloride should be suitable.
Peladow, Dow mini-Pellets, Prestone Driveway Heat, Kent Turbo Calcium and other dehydrated calcium chloride products are more potent than Dowflake. The dehydration makes them both more potent on a weight basis, and more dense, so they are much more potent on a volume basis. The problem is that it is rarely clear how much moisture is in them. Peladow specifies 90% calcium chloride minimum, but it may be higher in some cases. Dow Mini-Pellets at 94% minimum, but actually has a lower bulk density than Peladow. The best guess of an amount to use is based on the hydration levels and bulk density provided by Dow for these products. Using these numbers, I suggest that aquarists use 20% less VOLUME of the dehydrated versions in the recipes than is called for in Dowflake. So a recipe calling for 5 cups of Dowflake would use 4 cups of Peladow, Prestone Driveway Heat, Kent Turbo Calcium, etc.
Choosing other unknown brands of any of the products may be fine, or not. Iââ"šÂ¬Ã¢"žÂ¢ve not tested them for purity.
