This information was first posted in one of our "Questions of the Week."
Water changes improve Koi growth rates due to diluting pheromones. Water changes dilute toxins, reduce nitrates, decease DOCs (Dissolved Organic Carbons), reduce TDS (Total Dissolved Solids), and increase water clarity.
A goal of water changes should be to never change the final pond water temperature more than 5 degrees higher or 2 degrees lower than it was to start. While water changes have lots of benefits, Koi become stressed when the temperature shifts more than 5 degrees higher or 2 degrees lower in a 24 hour period.
Thus, FPWC (that's Frequent Partial Water Changes) are one of the very best things we can do on an ongoing basis to keep our ponds and Koi healthy!