It's obvious; fish breathe water through their gills. So, it stands to reason that you need to keep it clean, right?
Koi, even though they can actually breathe air, are no exception.
Plus, as bottom feeders, they do tend to muck up the water, which means that most of the time you can't even see them, and that's most of the enjoyment of having koi or other fish in your pond.
So, filter the water!
This does two things; one, it takes out any larger particles of organic matter that the fish disturb from the bottom of the pond, and second, it adds more tiny molecules of oxygen, which benefit the fish when they breathe it.
There are only a few parts to making this pond filter, which you may or may not have already. I'll bet you have a big 20 liter bucket in your shed.
Make sure it's a food grade one, not one that had paint or other chemicals in it. One from the deli or ice cream parlour is best and much easier to clean.
You can get special crystals or spray which are harmless to fish to make sure they have no residue in them.
The next thing is a pond pump to lift the water from below the surface of the pond up to the filter. Get one that is strong enough.
The final thing is lava rock. I generally use the kind that is meant for landscape mulch, but as long as it's in big enough pieces, the type for barbecues will work also.
So now you've assembled all your supplies, here's how you make the filter;
Drill holes near to the bottom of the bucket using a drill size smaller than the smallest pieces of lava rock that you'll be using.
You can start about one to two inches from the bottom, several lines deep.
This is to allow the cleaned and filtered water to flow out.
The definitive number of holes will depend on the size of your pond pump. The larger the pump in terms of gallons per hour, the more holes you will need.
The amount of water going into the filter should equal (approximately) the amount draining out of the bottom.
Dump the lava rock into the bucket, and rinse well to get any dust or small particles out (before it goes into the pond).
Drill a larger hole in the lid of the bucket to insert the hose of the pump. Lots of bucket lids have a hole already in them, so if it fits, omit this step.
Place the bucket higher than the koi pond, and hide it with some larger rocks, a decorative pot or a bin made of wood. The water should have some way to drain back into the pond, such as a waterfall.