Nitrate, NO3-N in your pond
Nitrate is the final product from the breakdown of ammonia released by the fish.
Nitrate is not especially harmful to freshwater fish but is a potent plant fertilizer and can contribute to the growth of unsightly and unwelcome algae, such as green water or blanketweed. Ideally, the levels of nitrate in the Koi pond should be controlled to help reduce the likelihood of these unwelcome algae blooms occurring. It is recommended that a Nitrate Test Kit is used to determine the quantity present in the pond and control the concentration through water changes.
Nitrite in your pond
As the ammonia in the water begins to reduce, the secondary break down product, nitrite will begin to increase and this is also very poisonous to fish.
Nitrite is a skin irritant and will cause the fish to display symptoms of irritability such as rubbing themselves, jumping, or even skimming across the surface of the pond. These symptoms are also commonly associated with parasites and it is sensible to eliminate nitrite as the cause before treating the pond.
Nitrite also has a rather sinister effect on the pond fishes blood, as it will bind very tightly