If you have been reading some of the pond and Koi forums out there some know-it-all hobbyist(s) advocate that the AFS is nothing but a cesspool of muck and detritus. If not cleaned like other filters more than twice a year you will end up with all kinds of bacteria problems and water degradation along with sick fish.
These are the same people that know nothing about the AFS, have never read about it or become the self proclaimed aquatic microbiologist that really contribute nothing to this hobby except misguided information to others. Their real agenda is never really understood by most but one thing is for sure: If an audience will listen then they will preach their sermons on that proprietary knowledgeable soapbox forever.
Despite the fact that I have said it several times before the AFS really doesn’t get that dirty if a proper prefilter is use before any pond water goes into the filter. Plus the fact that any detritus that does settle in the AFS never really gets compacted like filters that pass water through a medium, unless pond maintenance is forgone far too long. Most turns into mulm by a chemical reaction called oxidation anyway by the end of the season and becomes inert. This loose layer of mulm is nothing to worry about and is unsightly but nonetheless is a natural process of life in a pond.
It may consist of cat litter that has migrated out of the BCB’s, dead cyanobacteria, algae, and geographic pollution from outside sources. This could be from birds, trees and just dust and dirt that is airborne getting into the filter itself. All of what I have said sounds very logical if a well designed prefilter is use before the AFS. I mean, how else would the filter get dirty?
This year I did an experiment to see just how much dirt and detritus would really fill an AFS if all outside insults were lessen or eliminated altogether. This experiment would show that the amount of detritus would basically be plant and/or bacteria origin related and basically the AFS does stay very clean from one cleaning to the next.
As we know the BCB’s do not clog so that’s one less thing that has to have servicing throughout the year like other filter need. So the experiment was to keep a plastic 6-mill cover over the AFS all season long and at the end of the year at the October cleaning see just how much detritus really would accumulate inside the filter. Basically the filter would be a greenhouse for the aquatic plants with four 6" (152.4mm) holes in it, two on each side.
As everyone knows I have nothing but a homemade prefilter for my pond. Nothing expensive bought out of a box and is serviced once a week during high feeding season and once every fortnight when only feeding once a day. The cover has been on since last cleaning in May of 2014 and was remove in October of 2014 to be evaluated for mulm or detritus accumulation.
In October all aquatic plants are cut down and the filter is emptied and cleaned for the winter months ahead. This gives the Koi fresh water and a nice new clean environment for their long winters nap.
What I found out was, there was very little mulm or detritus in the filter this year at all! Once the outside insults were lessen to a greater degree the AFS remained very clean despite what some think. Most of what did exist was plant related along with a little Goldfish waste, but was nothing to get upset about. I was really quite a surprised at just how much outside pollution gets inside an AFS. If pond husbandry is carried out on trimming any dead or dying plant foliage then pollution inside the AFS is very negligible.
You also must remember that algae and cyanobacteria are part of the equation of pollution that gets inside an AFS and dies and then is broken down into mulm. Plant DOC’s are really nothing to worry about unless in excessive amounts. In most ponds the DOC’s come from animal related insults about 90% but not so much plant matter. If you can control the amount of insult getting into an AFS then the cleaner it will be between seasonal cleanings.