Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Sources of fertilizer in a low tech set up:
Fish food. Digested by the fish or fallen to the substrate and digested by bacteria. Good source of N, P and traces. Not so good for K, Fe or C.
Water changes, depending on your tap water.
GH >3 German degrees usually means there is enough Ca and Mg. If there is any doubt then a separate Ca test would sort that out. If the water is softer, then use a GH booster. Natural water with a somewhat higher GH often has other minerals such as K and traces. Not always, though. Some water has Fe in it, but not always in a form the plants can use.
Decomposing leaves, and carbohydrate fish food: Good source of C at the levels needed in a low light tank. Just barely enough, so supplementing C might be one of the first things you do.
Added fertilizer: The sources listed above are often too low in K and Fe. I would start by dosing those, anyway. And C. Excel might be a reasonable source of carbon. Fertilizer tablets under the soil would be a very good way of ensuring the rooted plants have fertilizer.