There are many articles on how to keep your koi pond water clear and healthy. There is always talk about UV lights which is correct but UV lights only take care off algae in the water. All other debris like excrement, dust, leaves and all other air born dirt has to be filtered. When you have over stock of koi in your pond the maintenance, backwash, stirring of sand and cleaning of pump filters becomes a nightmare.
There are three items that minimise your maintenance and is a absolute necessity when investing in a koi pond: Bottom drain to a Settlement chamber, and a Bead filter system with a blower to stir the plastic beads and the obvious bio filter and UV lights.