Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Seachem bottles N, P and K in separate bottles so you can customize the dosage for your tank.
If your tank is more of a low tech tank, with moderate light, no additional carbon (CO2, Excel) then fish food might supply enough of most nutrients.
Fish food is high in N, P and most traces. If the NO3 tests show that the fish food is supplying enough nitrogen, then it is also supplying enough phosphorus and most trace minerals.
Fish food is low in potassium, iron, calcium and magnesium.
If you are relying on fish food for fertilizer, then the first elements to supplement will be potassium (Seachem Flourish Potassium) and Iron (Seachem Flourish Iron).
Water changes will usually supply enough Ca and Mg, as long as the GH of the water is at least 3 German degrees of hardness.
If your tank is mostly fertilized with fish food, then I would not supplement with Seachem Flourish Comprehensive. I would target the minerals in short supply.
I would not mix this with water that is left standing around for a week.