The 30% limit on water changes is primarily to protect koi health and minimize water parameter shock. Sudden, large-scale water replacement can cause drastic shifts in temperature, pH, and hardness, potentially leading to "temperature shock," which may result in koi convulsions or refusal to eat. Furthermore, the influx of new water can wash away a critical portion of the beneficial nitrifying bacteria responsible for breaking down toxic ammonia and nitrite. If these bacteria are depleted faster than they can reproduce, the system's biofiltration collapses, allowing waste products to accumulate rapidly and pollute the water.
Long-term excessive water changes can also strip away the protective mucous coating on the koi's skin, making them more susceptible to bacterial and parasitic infections. Therefore, it is recommended to limit each water change to 20–30%. The new water should be dechlorinated (aged or treated), added slowly to avoid strong currents, and ideally matched closely to the pond's temperature and pH. This practice safely replenishes essential minerals while allowing both the fish and the microbial ecosystem to adapt gradually, maintaining long-term stability.
