Cleaning a Koi pond doesn’t have to mean taking out the fish, draining the pond, and refilling it. It can be as simple as draining only half the water and not removing any fish!
Take a fine mesh net and attempt to net out debris from the water. If the water has a foul, rotten egg odor when you start, stop. You will need a kiddie pool with a net cover and either an air stone or small pump to oxygenate the water. You will have to remove the fish to the temporary container while you drain the pond and remove debris. Make sure the water in the kiddie pool is dechlorinated. Net the fish and place them in a large bowl that will float on the water of the kiddie pool. Cover the bowl with a towel and let it float or hold it in the water for 15 to 20 minutes (with an air stone) for the fish to be acclimated to the water. Then, place the fish into the kiddie pool with a netted cover and an air-stone or pump while you clean the pond.
If the pond does not have foul rotten egg smell, use a net to take out debris on the bottom. Drain about 25 percent of the water and refill it using a dechlorinator. Tap water is about 50 degrees, so add water slowly to acclimate fish to the temperature change. Use an air-stone or small pump to oxygenate the water.
During the next day, drain another 25 percent of the water and refill it using the same technique while removing debris you missed.