Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Fish food generally has a fair amount of N, P and most traces. So if the NO3 was mostly from fish food, then I would make the assumption that P and traces are OK.
Fish food is low in K, Fe, Mg and Ca.
Water changes generally take care of Ca and Mg, unless you have really soft water. But you could supplement with a little GH booster, see if that helps.
I would definitely add K and chelated Fe as the first step to dosing a tank that gets most nutrients from fish food.
What are the ingredients in the fertilizer you have added? If it includes NO3, then this sure could contribute to the rise in NO3. Fish seem to tolerate NO3 from fertilizer better than from decomposing fish food. If this NO3 is mostly from fertilizer, then I would not do extra water changes. Just let the plants deal with it, and do regular water changes, whatever schedule you are used to doing.
Good idea, though to look at the overall picture. See where the plant nutrients are coming from, and get some sort of idea if they are balanced or not. Then look into supplementing the nutrients that are in the shortest supply.