Ammonia Test Kit

Ammonia is the main waste product produced by fish as a result of feeding. It can also be generated by the decay of uneaten food or decaying organic matter, such as plant leaves. It is invisible to the naked eye and its presence or absence cannot be determined by looking at the water alone. It is extremely toxic to the fish but is broken down into less toxic nitrite by the beneficial bacteria found in the filter. 
There is no occasion where any ammonia is tolerable. A level of zero mg/l (ppm) should be maintained at all times.