I would like to clear up some misunderstandings about cleaning filters, why, how and when. I have used all kinds of filters on indoor tanks and ponds, for many years, and have a good understanding of how they work, how to use them successfully, how to maintain them, even how to build them. To understand how various filters work, you need to know a bit about the nitrification cycle in addition to understanding what you want your filter to accomplish. To simplify: Fish produce ammonia as a waste product. If they are alive, they are excreting ammonia, even when not being fed (as in pond fish during a winter fast). They also produce solid feces, and if left in the tank, these feces break down and produce more ammonia, as do decaying plants, uneaten food, undiscovered dead fish and infertile eggs left in the tank. In any fish tank or pond, even one without filtration, bacteria that consume ammonia grow naturally. In other words, you do not have to add them, they just exist - the come in from the air, the water, the fish, plants. They cling to all surfaces in the tank and will grow a colony large enough to consume all the ammonia produced as long as there is enough surface area for them to live on. If more ammonia is introduced, as in adding fish, fish growing larger and/or feed increased, the colony will expand. If less ammonia is produced, some bacteria will die off. They regulate themselves this way, and in an established tank the changes are virtually imperceptible by us. These ammonia eating bacteria create nitrite as a by-product. Once nitrite starts appearing in the tank another type of bacteria start growing (again, appearing out of nowhere and clinging to all surfaces) which consume nitrites and convert them to nitrates. They form a colony as well which co-exist with the other bacteria. There are no bacteria that grow naturally in a home aquarium or pond that consume nitrate ... for our purposes, nitrate needs to be controlled by either water changes or perhaps using enough live plants that they take up the excess nitrate. Water changes are important for other reasons, so usually controlling nitrate is not an issue. Back to the filters: Basically, there are mechanical filters, biological filters, combinations of these (most of those available for home aquaria are combinations) and another category might be fines filters. Mechanical filters are used to trap and filter out solid waste and debris. Biological filters contain media that provides large amounts of surface area for bacterial colonies to grow on. Fines filters give a final polish to the water before sending it back to the main body of the tank. So let's look at some common filters: An HOB (hang on back) filter sucks water through an intake tube usually from somewhere in the bottom half of the tank and sends it through either a foam block, possibly some media in the chamber and/or a cartridge that holds dense fibers and sometimes a layer of charcoal. This is primarily a mechanical filter, although with a sponge or other media in the chamber it provides some biological filtration as well. It is meant to suck up solid waste, trap it in the sponge or cartridge and send clean water back to the tank. If you don't clean the resulting mulm (dirt, sludge, muck, debris) out of the filter chamber, sponge or cartridge regularly, what happens to it? It decomposes, as it is sitting there, and creates more ammonia. I am going to emphasize this - there is no point in sucking it up and trapping it in the filter if you are not going to then remove it. It is still in the water column and still polluting the tank, just as much as if you had left it on the bottom. In fact, with swiftly moving water constantly pushing against it in the filter, it decomposes even quicker, and crates small particles that get through the sponge or cartridge and back into the tank. Cleaning your HOB depends on how it is set up and it's intended use. Due to its size, it clearly does not provide an abundance of biological conversion, but if you have sponges or media in it, it does provide some, and if you have a large tank (lots of surface area on the glass), low stocking levels, a shallow layer of gravel or stones on the bottom (more surface area), and even plastic plants and ornaments, you may not need more from your filter. You may also have additional biological filters to take up the slack, leaving the HOB to be purely mechanical. So - if there is a sponge to trap solids - rinse it regularly to remove those solids. The more often the better. If your tap water is treated (chlorine or chloramines as opposed to, say, well water) use dechlorinated water or fish tank water from your water change. Running water or squeezing and swishing both work. If there is biological media in the chamber (little plastic balls or tubes, ceramic or stone-like bits, things like that) rinse them - they will work better if not covered with mulm. If all you have is a "disposable" cartridge, you can do several things. One is rinse it over and over till it falls apart, then replace it, another is to take it apart and put something more durable in the frame, like a thin sponge, or, to get the most out of your HOB, fit in some biological media or a sponge into the chamber before the cartridge and consider the cartridge the final "fines" filter, and replace it when necessary. Before I move on to other types of filters, I want to make an important point, one that causes the most confusion, and causes people to advise NOT to rinse your filters often. Beneficial bacteria do not live free swimming in the water and they do not live in mulm. They live on hard surfaces (including the plastic that a sponge is made of) and, in fact, cling tenaciously to these surfaces. They cannot be rinsed off with normal tap pressure, even vigorous scrubbing will not make a serious dent in their numbers. As proof, I offer the concept of the moving bed filter, common in high end koi ponds. A barrel is filled with "Kaldness" media, which is a small plastic tubular media, each piece having ridges and wings providing lots of surface area. Pond water passes through the barrel, and the media is kept in a constant rolling boil with large air pumps. The scrubbing action is enormous, and in fact, the claim is that old, dead bacteria are rubbed off this way, making room for fresh, new microorganisms to grow on the media. This is considered one of the best and most efficient biological filters for ponds that are dealing with fish 2-3 feet long and feed measured in pounds per day. There are huge amounts of ammonia to process in these ponds and, believe me, these filters harbour the bacteria perfectly well, despite the banging and constant rinsing they are subject to. I have always rinsed my filters once to twice a week, and have never experienced an ammonia or nitrite spike afterwards. As long as you are not killing your bacteria with chlorine, they will survive any amount of rinse offs just fine. If you are still worried, just gently swish all of your filter components in a bucket of tank water - the mulm and debris will fall off, the bacteria will stay. Sponge filters and sponge walls: Sponges are dual purpose. They are usually set up to suck water through them using powerheads or air bubbles, trapping solids on their outer surfaces, while also providing a great amount of surface area per square inch for bacterial colonies. However, the more clogged a sponge gets with mulm, the less clean surface is available for bacteria, and their numbers can be reduced by essentially being oxygen starved. Sponges should be squeezed out frequently - the solids will be removed from the water column and the bacterial colonies will work more efficiently. Canister filters: These usually have different layers inside that provide mechanical (layers of different density sponges), biological (some kind of specialized media in baskets) and fines (a final piece of fine sponge or filter floss pad). It amazes me that people leave their canisters for months at a time before cleaning. And then describe them as disgusting when they do. Again, if you don't take them apart and rinse the components, it is as if the solid waste were still in the tank! Now, canisters are very efficient, and have a lot of room in them for biological activity - a much greater volume of media can be had compared to most all-in-one filters, and, depending on how many fish you have, how much water, etc, they can probably easily handle the extra ammonia produced by decaying fecal matter and debris trapped inside for weeks on end. But, and here is my whole point, keeping filters rinsed properly reduces the total ammonia levels in the tank (by removing solids before they decay further) and creates a more efficient environment for beneficial bacteria to grow and consume pollutants in our tanks. This is good insurance for fish health, allows higher stocking levels if you so desire, reduces odors and unsightliness around the tank area and is simply good husbandry.