In a lightly planted tank with several fish, good feeding, and hardy, slow-growing plants, the need for additional fertilizers is probably nil.
Fertilizer requirements depend on what you are growing, how fast plants are growing, and your stocking level. In a lightly planted tank with several fish, good feeding, and hardy, slow-growing plants, the need for additional fertilizers is probably nil. For large, root-heavy feeders (such as Amazon swords), it may help to add fertilizer balls or root tabs to the substrate. If you have filled your tank with plants that grow fast, have some aerial roots, are epiphytic (plants that attach to surfaces and are not planted) or float, and your stocking is light to moderate, then liquid or powdered fertilizers may be of assistance.