Koi Pond Filtration:
Clear water does not necessarily mean clean water, it may contain colorless impurities, such as ammonia and nitrite, that are harmful and even lethal to Koi
Cleaning your Koi pond filter.
Filters need to be cleaned occasionally to remove sediment, take this into account when building one. Add a bottom drain to each filter chamber so that sediment can be let out, it makes cleaning much easier. Another thing that makes cleaning easier is to put filter medium in net bags, not just pour it in all at once, as it can then be more easily removed, one bag at a time.
One last important thing, never ever put tap water in a mature filter, it will kill all the bacteria and you will have to let it mature all over again.