Nitrite

From Koipedia

NitrIte, not to be confused with Nitrate, is the middle step of the Nitrogen_Cycle.

Nitrite levels should be low to non-existant in a healthy fish pond. Nitrites can be made less toxic to Koi by adding some salt to the water, somewhere between 0.15% and 0.3% salinity would be good for this reason.

If noticeable Nitrite is present, corrective measures should be made to maintain fish health.