Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
KNO3 is used to supply nitrogen. If your tank has a lot of fish you might not add much of this. Fish food might supply enough nitrogen that you need only a little KNO3.
If you add a lot of KNO3, then that is enough to count as a supply of potassium, too. But if you are not adding very much, then you are not adding enough K.
KH2PO4 is a source of phosphate. You will not use much, so it does not count as a source of K. You might not need any, if the fish food supplied enough.
K2SO4 is a source of potassium. If you need only a small amount of KNO3, then you might need to add more K, and this is the most common form.
Epsom salt is a source of magnesium. If your GH is low you might also need calcium. Plants use Ca and Mg in a ratio of something like 3 or 4 parts Ca to 1 part Mg.
I would not dose Epsom salt to correct low GH without also testing the Ca. You might need to dose Ca as well as Mg.