To begin with, calling a Bio-Filter a filter at all is a misnomer. The proper term is Bacteria Contactor. Many people confuse this in thinking that a part of the job of a bio filter is to filter solid fish waste. This devise is actually the living quarters for a whole bunch of different bacteria that need to eat. The things they eat are the ammonia and nitrite that are generated in a fish system that you do not want. Each bacteria is a part of a chain which takes ammonia, which is the element that fish waste creates and they turn it into a different form. The first bacteria in the chain consume the ammonia and they excrete nitrite. The second bacteria colony consumes the nitrite and excretes nutrient. The purpose of the device known as the bio-filter is to provide the area where the bacteria can accumulate and then have the effluent from the re-circulating system flow pass them so they can feed. Since the bacteria will stick to almost anything, the media that has been used by system designers has been everything from plastic shotgun wads to plastic hair curlers. Some companies make special coring just for this purpose. For over twenty years we at Global Aquatics have studied the various designs for biofilters, the media, the applications and water flows through the filter and past the bacteria. What we found was that although the filter would work for awhile, in almost every case, sooner or later the media became clogged with very small suspended particulate from the fish waste that was not removed prior to going to the filter, and one of two things happened. (It is almost impossible to remove 100% of the particulate prior to biofiltration). First the waste would begin building up on top of the media and sometimes along the sides of the vessel. At this point the denitrifaction bacteria started to loose clean places to live and their population began to drop. Next the waste would begin to attract aerobic bacteria which would deplete the biofilter of the oxygen the denitrification bacteria needed for survival. In time, the biofilter no longer served it's intended purpose at all, but instead became a BOD (Bio Oxygen Demand) generator. This action not only failed to remove the original ammonia and nitrite from the water, but actually created more as it digested the particulate. The obvious answer to this problem as described by the designers was, You need to back flush it periodically to clean the waste out The problem with this theory is that it can not be done without also removing the denitrification bacteria as well. Now you have an inactive biofilter and by the time the bacteria are able to rebuild their population back to the required numbers it is time to clean the filter again. There is only one thing that can be done to solve this problem, that is to have the best possible filter to remove even the smallest amount of solid particulate. If the only thing entering the bio filter was clean, ammonia filled water the bacteria would consume all of this with little problem. Removing all of the solids from the water column also cuts down on the ammonia load because once it is removed from the system it can no longer decompose and make even more ammonia. For this reason I tell people that the best working bio filter is one that has the best solids filter ahead of it. It was for this reason that we developed and refined our auto-cleaning drum filter.