Don't forget to check your oxygen. Many Koi fish owners don't realize how important maintaining the proper level of dissolved oxygen in their pond really is. They also don't realize how factors can affect the oxygen level. Simple aeration can help make this easier than it may sound.
Carefully watching the oxygen level is necessary for several reasons. Certainly we know the fish need good oxygenated water. Stagnant water is certainly unhealthy. But proper oxygen levels are also necessary for the health of any vegetation you have in the pond and is critical to the chemical processes that go on continually. Every pond has its own oxygen demand. This is based on the number and size of fish you have, the amount of vegetation in the pond and the amount of plant decomposition that is going on.
The water itself absorbs oxygen differently whether it is warm or cool, moving or standing. The salinity of the water also affects its absorption capability but since our ponds are freshwater ponds we don't really have to worry about salinity.
It is during the warm months that Koi pond aeration becomes so important. Be sure to check the oxygen level frequently when it gets very hot. Keep the level at least between 5.0 and 5.5 mg per liter.
It is important to keep the water moving. Water that is moving has a much higher oxygen absorption rate. Oxygen levels can drop quickly in water that is just standing. Using a fountain or waterfall will help a great deal.
Algae growth is another thing to watch. Don't let it get out of hand. During the day algae produces oxygen which is good, but at night it will deplete the oxygen. Too much algae will make it more difficult to control the oxygen levels.
Pick the coolest part of the day to feed your fish. When koi fish are feeding their respiration rate increases making them utilize more oxygen. Feeding them in the heat of the day, when the temperature of the water is high and oxygen levels already lower, will only make matters worse. Oxygen levels can drop very quickly in these circumstances.
Always have good pond aeration. If you are putting in a new pond, plan for good water circulation from the start. For an existing, pond think about adding fountains or other water features that will improve the overall water circulation.