Do your koi get enough oxygen to sustain them?
 
If you see them hanging out by the waterfall or at the top of the pond gasping for air then the answer is no!
 
A koi pond needs aeration to keep it and the koi healthy. Fish need oxygen to survive. They breathe the oxygen dissolved in the water.
 
Lack of it will result in the death of your koi. You must get that needed oxygen into the water. To do this you need a system for aeration.
 
There are two factors that affect the amount of oxygen in the water:

1.   Algae growth which depletes oxygen level
2.   Too many fish for the size of the pond.
 
Either situation requires oxygen supplementation.
 
One way to aerate the pond is to add a simple waterfall. Besides being pleasant to hear and to look at, the splashing of the water also increases its oxygen content.
 
There are bubbling devices like airstones and rings that you can use if constructing a waterfall is not possible. An air pump is needed to use these devices. These are especially useful in aerating the bottom of the pool that is farthest from the surface and receives less oxygen.
 
Fountains are another good way to get oxygen into the water. In fact, anything that breaks the waters surface introduces oxygen into your pond.
 
The best way to ensure enough oxygen would be to combine air stones with a waterfalls or fountain to get plenty of oxygen to every level of your pond.