It is necessary to have a large amount of aeration in a Koi pond, and this is usually supplied by a venturi, and sometimes a waterfall. If the pond is well stocked as usually happens when the fish get bigger then many koi-keepers find it necessary to add extra aeration with air stones.
This does not look very good in a pond and to overcome this many dealers have come up with air diffusers which are fitted instead of the usual dome on the top of the bottom drain.

This photo shows a typical PVC bottom drain but in this case it is fitted with a diffuser instead of the normal dome.
These diffusers were first introduced into the UK by Prokoi Enterprises, but several different types are now available.
They present a circular column of fine bubbles from the dome to the surface of the pond, and it is claimed that the movement of the water helps to move the debris towards the bottom drain and so improving it's efficiency.

It was normal practice to put the airline inside the 4" pipe for the bottom drain, but as this can cause silting up of the pipework it is now normal practice to run the airline in a separate pipe.

This is a close up of the above bottom drain and it can be seen here that an additional pipe connection is fitted so that an extra pipe can be installed to carry the airline for the diffuser.