Koi pond filter consist of mechanical, biological and chemical chamber. In the biological chamber must not have debris as dirty particle.But Even biofilters get dirty. This can be in many forms and of many descriptions. In the most fundamental case, 'dirt' is macro debris that is carried to the biofilter from the greater pond environment. This can be leafs, seeds, acorns, fish pellets , insects and feces. These really should never get to the biofilter ( which is more accurately called a bio-reactor and not a filter at all) as every pond should have a first line of defense in the way of a skimmer basket and pre-filter. A prefilter is a concept more than a piece of equipment. The WAY your prefilter words and how efficient it is, further describes want you prefiltration approach is.
