Ammonia

From Koipedia

Ammonia, is the first step of the Nitrogen_Cycle, and is created from Fish waste, rotting fish food, leaves and other organic material.

Ammonia is very harmful to fish, and should be kept at 0 in a healthy fish pond.

Ammonia is broken down in Biological-filtration systems, and turned into Nitrite, and from there into much less harmful Nitrate.