Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Fish food contains most of the elements that plants need, but not always in the right ratios.
Fish food is low in potassium, iron, calcium and magnesium.
The proteins in fish food get digested by fish, and decomposed by microorganisms and enter the water as (among other things) ammonia (NH3) and ammonium (NH4).
Different organisms remove ammonia and return nitrite (NO2) to the water.
Different organisms remove nitrite and return nitrate (NO3) to the water. This nitrate is not combined with anything else.
Plants can use all these nitrogen containing molecules.
Fast growing plants can use up all these and need more.
The NO3 test is a good way to tell if the plants are using all the nitrogen from fish food, and if they need more.
One common way nitrogen is added to the tanks is in the form of KNO3.
This supplies potassium (which fish food is low in) and nitrogen.
If your NO3 test shows that the plants are not even using all the NO3 from the fish food your plants would still need a supplement that provides some potassium.
K2SO4 is a common fertilizer that supplies potassium without the nitrogen.
Fish food is low in calcium and magnesium. If the water you are using for the tank has a GH of at least 3 German degrees of hardness, then there is probably enough Ca and Mg. If you think there is a problem, then dose a GH booster that contains both Ca and Mg. Do not dose JUST Mg (Epsom salt or other) unless you have figured out that the water has enough Ca, and is only lacking Mg.
Fish food is low in iron. A chelated iron will remain available to the plants for the longest time. Simpler forms of iron get locked up pretty fast, so are less useful in the tank.