The nitrates are then removed by algae and plants which completes the nitrification process in aquaria and ponds. They also can be removed by frequent water changes. Koi growth can be stunted by high levels of nitrates.

Unfortunately, many Koi have been lost because of total ignorance or a poor understanding of this process. What is not well understood is you can completely destroy a great biological filter by rigorously cleaning it with chlorinated tap water, and throwing out the media and replacing it with new media. Some chemical treatments can also destroy it.

High levels of nitrite lead to brown blood disease. Brown blood disease occurs in fish when water contains high nitrite concentrations. Nitrite enters the bloodstream through the gills and turns the blood to a chocolate-brown color. Hemoglobin, which transports oxygen in the blood, combines with nitrite to form methemoglobin, which is incapable of oxygen transport. Brown blood cannot carry sufficient amounts of oxygen, and affected fish can suffocate despite adequate oxygen concentration in the water. This accounts for the gasping behavior often observed in fish with brown blood disease, even when oxygen levels are relatively high.  

Sodium chloride (common salt; NaCl) is used to "treat" brown blood disease. Calcium chloride also can be used but is typically more expensive. The chloride portion of salt competes with nitrite for absorption through the gills. Maintaining at least a 10 to 1 ratio of chloride to nitrite in a pond effectively prevents nitrite from entering Koi. Where Koi have bacterial and/or parasite diseases, their sensitivity to nitrite may be greater, and a higher chloride to nitrite ratio may be needed to afford added protection from nitrite invasion into the bloodstream. As a general rule,  strive to maintain at least to 50 to 100 ppm chloride in pond waters as "insurance" against high spikes of nitrite concentration. 1,000 ppm of salt is equal to a 0.1% level.

Brown blood disease can be prevented, or at least minimized, by close monitoring of nitrite, chloride, and total ammonia nitrogen (TAN), and by maintaining the proper chloride to nitrite ratio. If brown blood disease does occur, the condition can be reversed by adding salt to the water. Koi surviving brown blood disease or nitrite stress are more susceptible to bacterial infections, anemia (white-lip or no-blood), and other stress-related diseases. These secondary problems, such as Aeromonas or Columnaris infections, often occur 1 to 3 weeks after brown blood disease occurs.