Ammonia is one of the most important parameters to monitor in aquariums because any amount can be toxic to livestock if it is not addressed right away. Ammonia is commonly elevated in new tanks while establishing the beneficial biological filter. High ammonia can also be present in established aquariums from lack of maintenance, overstocking, or when the bio filter has been interrupted by medication. Ammonia can reach lethal levels very rapidly, and should be tested immediately in the case of fish illness or death. Monitor ammonia levels closely when using any medications or treatments.

The ammonia level in any tank should ideally always be at 0 ppm. Symptoms for acute and chronic ammonia poisoning include: heavy breathing and gasping at surface; clamped fins; discoloration; increased occurrence of disease; death.