Filters should function as efficiently as possible at all times.
Because it is unlikely that all solid waste will be trapped in settlement, the biological part of a filter system must be well designed to minimize blockage of the media.
If a blockage occurs in a filter then it must be cleaned out, and much of the valuable bacterial bed that has built up over, may be, a period of years will be lost in the cleaning process.
When the filter is subsequently returned to duty, it will have a greatly reduced capability.
Water quality will therefore fall and you must take some form of corrective action to avoid or minimize this potentially dangerous time for the koi.
Using open filter media is one way to keep biological filters from blocking.
Another way is to direct the flow of water through the media in such a way that they are easy to clean.
Modern filter systems incorporate both these ideas to minimize any possible risk of blockage.
Flow through the media in all types of filter systems is normally upwards, but where aquatic foam is used in a chamber, the water would normally flow downwards.
The foam used in filters is often graded for its void size and stacked to provide for both mechanical and biological action.
Void size relates to size of the pores, or holes, within the body of the foam.
Make sure you can remove the foam layers easily for cleaning.
An advantage of up flow is that in a well designed chamber floor, the settled waste rests at the bottom of the chamber, rather than lying on the surface of the media.
From this low position it is easy to flush the waste through the chamber's own bottom drain to sump.
If necessary, back flush the media by isolating the chamber with valves or standpipes.
Once isolated, open the bottom drain valve.
Much of any accumulated waste lodged in the media will wash down the drain and away to sump with minimum effort.
Gently hosing with pond water can help remove any stubborn dirt.