fertilizer or nutrient? if u want to add a fertilizer indoors I'd say do it when u mix your soil, adding it after (say ontop of the soil) could attract unwanted pests. but if it is a nutrient u want (aka somethin to mix in the water) I'd say get urself a NPK around 24-7-10 or 20-10-10 doesnt much mater as long as its high in N and rounded in P and K. predy much any Grow nute at 1/4 strenth works fine. If ur looking for a fertilizer to add to the soil I'd look at worm castings or bat guano