Ammonia is excreted by fish directly into the water, and is also released from the breakdown of solid waste and uneaten food. Ammonia is highly toxic to fish, but is usually removed from the water by the bacteria which live inside the filter. However, in new ponds, or if the filter bacteria are damaged, the ammonia level may become toxic. The only way to know what the ammonia level is, is by testing it with an Ammonia test kit. Regular testing, especially in new ponds, will prevent damage caused by ammonia poisoning.