Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Fish are not the source of fertilizers. Fish food can be, but it is pretty expencive and when it rots in the tank is not pretty.
Soil may or may not be a source of any of the elements the plants need. It can be, but the plants use up those elements if they are not constantly being added.
A good soil will hold the nutrients available for the plants, research cationic exchange capacity.
But, like nice dinner plates, the soil needs to be refilled.