Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Fish food is a pretty good supply of most fertilizers. You can judge by the NO3 test.
Fish food does not supply enough potassium, iron or calcium.
You have two possible sources of potassium so far:
If you need to use GH booster for the fish, many of these contain potassium (Equilibrium, Barr's GH booster) as well as calcium and magnesium.
Check the tablets. If they contain potassium and iron, then I think you do not need to dose anything except carbon. Excel is good for that.