“Did You Know? KH (Carbonate Alkalinity) levels need to be at least 80, and preferable between 100-300 ppm for continuous nitrification (ammonia removal). Phosphate levels should be 0.050 to 0.100 ppm, the lower the better. Water changes and plants will remove phosphates from pond water. The pH for a koi pond can be 7.2 to 9.0 and the koi will thrive, as long as it is stable at that level. (Mine stays at 8.8 – 9.0).
Even very minute amounts of copper can be toxic to koi.”
If you do not have a method to test water, now is a good time!