Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Plants need over a dozen elements to live.
Some are pretty simple, like hydrogen and oxygen.
Carbon is a good idea, but in a low tech tank the decomposing matter in the soil may supply it.
Fish food has the elements plants need, but may be low in potassium and iron.
When I wanted to improve conditions for my tanks the first ferts I added were C (Excel), K and Fe (Leaf Zone, a liquid fertilizer).
The other minerals that you should check is with the GH test. Plants and fish need calcium and magnesium. If the GH is too low (<3 dGH) these might be marginal. Water changes usually supply these minerals.
When you first start with a commercial product, I would try the low end dose, whatever the label says. If that helps, you could stay with that dose, or try a bit more.
What may happen is this:
When you supply the missing element, then the next element in shortest supply becomes the deficient one, so you should watch and dose that...
until you reach some other stopping point, such as the light.