first thing, seedlings need light and warmth to grow---especially if temps are cool (60f or less) seedlings will probably be slow so consider that (if you haven't already). assume your fish fertilizer came as concentrate in a bottle with some kind of instructions (?)---if so, use the lowest recomended dosage' when in doubt use HALF that. if nothing else, a teaspoon in a half gallon of water should be o.k. good luck.