Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
There are in fact various grades and purities. Typically you'll receive cheap agricultural grade from any seller, which is not particularly pure.
But still pure enough that it doesn't affect dosage, and the small amount of impurities in an already small dosage have no effect on an aquarium. Even if, for example, you were to find a lot of black specks in a batch of white crystalline powder - which I have seen. It still worked the same as any other batch.