Toxicity of potassium and magnesium?

Hello fishfolks,

I wondering if any of you out there know the toxicity of either potassium 
sulfate or magnesium sulfate. Specifically, up to what levels can these 
substances (Mg, K and SO4 ions) be considered safe in an aquarium. This 
question does not seem to have been addressed in the archives.

Thanks in advance
James Wong 
u2120863 at acsusun_acsu.unsw.edu.au