I don't know your level of knowledge obviously, so bear with me. Nitrogen occurs in 3 forms in the aquarium, as ammonia/ammonium, nitrite and nitrate. A fourth, nitrogen gas, dissipates into the atmosphere and rarely remains in the tank. Aquatic plants use the ammonium as their nitrogen.oh i see, so ammonia could be a good source of N for the plants? but ammonia could burn our fish, right?