Good water quality is the key to a healthy, beautiful aquarium. Tap water contains chemicals, such as chlorine and chloramines, which make water safe for drinking, are highly toxic to fish. High levels of these chemicals destroy fish gill tissue, preventing the uptake of oxygen which can lead to fish death. Even low levels irritate delicate fish tissue. Tap water must be treated before adding fish to make sure fish remain healthy. API TAP WATER CONDITIONER Aquarium Water Conditioner instantly.