Koi water quality is the most important parameter for keeping healthy Koi. 

The only way we can monitor it is to test for the many variables that make up a comfortable Koi environment. It is a pain to rigorously follow a testing schedule, and many people leave a Koi test kit on the shelf collecting dust. Sometimes those people suffer the grief of losing their favorite Koi, and then they test the water and realize it was their fault. They let their Koi suffer because of their own laziness. 

The most devastating killers (after lack of oxygen) are ammonia and nitrite. Once the pond is set up and going we too often assume the oxygen level is fine, particularly since it is hard to test with any degree of confidence.

Chlorine less than .05 ppm
Oxygen more than 5 ppm
Ammonia less than .1 ppm
Nitrites less than .2 ppm
Nitrates less than 50 ppm
pH 6.5 -9
Alkalinity 50 - 170 ppm
Hardness 75 - 150 ppm
Salinity 0.1 - 0.3%
Phosphates monitor
Copper monitor