Good question. I'd like to know the answer, too.
Your test kit is probably working, but probably only measures ferrous iron, not ferric. Ferrous iron is the form most easily used by plants. (or I may have it backwards, sorry). Anyway, the form used by plants is quickly converted to the other form, unless chelated. This means you can add a bunch of the proper form of iron, and get a test kit reading right away, but you'll probably see 0 the next day.
So how much to add? There is the problem. Too much and you'll poison your fish.
Hope someone answers.