Mechanical filtration

Mechanical filtration operates like a very fine strainer, active down to an almost microscopic scale; however, bacteria and protists pass through, along with all solutes: in mechanical filtration water is not "purified" in any sense. Mechanical filtration media are fairly straightforward: most commonly, a pad or bag of close-woven synthetic fiber  is stretched over a rigid open frame to completely block the water flow. The water is forced through the medium, leaving its particulates, whether they're silty or organic, ensnared in the web. Other mechanical filtration media besides floss include sponges, baffles such as "bio-balls" and haircurlers, etc., fused glass or ceramics, even sand grains. Temporary micro-filters using diatomaceous earth can "polish" the water to a swimming-pool clarity, when critical guests are coming over. Diatomaceous earth is a good example of a just-about purely mechanical filter.
 
A general hobby-supplies shop will sell you rolls of 100% polyester quilter's batting that can be cut and used like filter floss or bonded filter pads as a much cheaper substitute medium for mechanical filtration. The batting comes in various "lofts" or thicknesses. It's bonded with a harmless and inert cured resin that is solely meant to keep fibers from migrating. Don't substitute cotton or cotton/poly batting: cotton will rot. If you hesitate to use a product not specifically packaged for aquarium use, get the "hypo-allergenic" batting. "Hypo-allergenic" is the phrase to look for, whenever you're looking for substitutes for captive-market aquarium goods. It means that the product is held to a high standard of freedom from chemical irritants that don't belong in the aquarium.
 
The mechanical filter should be the first phase, the "upstream" phase of your system, because particulates not trapped in the mechanical filter would clog the chemical filtration medium and the biological filter. Here is one inherent flaw in an undergravel filter: the gravel layer that forms the mechanical filter doubles as the biological filter.
 
In mechanical filtration, only suspended particulate matter is trapped, no matter how small those particles may be. Particulates are caught within the small spaces of the filter medium or get plastered against its increasingly sticky upstream face. Mechanical filtration operates physically, working like a sieve on a minute scale; it doesn't affect anything that is dissolved in the water. Besides fish or snail feces, plant debris and uneaten food, particulate matter includes suspended silt ("colloids"), which can make the water hazy.
 
Particulates that settle out in the tank won't get filtered. Detritus that accumulates in the upper surface of the gravel or among plant stems can be siphoned away, the prelude to a partial water change. What your siphon can't reach you can flush into the water column with a modest jet of fresh replacement water during a water change; then the mechanical filter can take it up.
 
When to clean the mechanical filter. Mechanical filters clog. Fine mechanical filters clog quickly. It's a well-worn paradox that "a dirty filter filters cleaner." As the upstream surface of the filter clogs with particulates, a biological film of bacteria begins to form. Smaller and smaller particulate matter is trapped in the brownish sludge that develops. An ideally dirty filter is almost as effective as diatomaceous earth in polishing the water free of floating particulates. Then, at a certain point the water flow begins to be seriously impeded. Filter "by-pass" describes water flow that passes unfiltered over the top or round the sides of the filter. In an open unpressurized filter, such as a hang-on-tank ("h.o.t.") unit, you can take by-pass as a sign that the mechanical filter medium has become too clogged to function, and it's time to rinse the filter medium— or throw it away. In an enclosed pressurized canister filter, reduced outflow is the sign to watch for.
 
Mechanical filter media should be gently rinsed and back-washed on a regular basis. (In my planted aquaria, where I feed a good deal of boiled spinach, I can go about a week.) But don't be too hasty about throwing the medium away.
 
"Out of sight, out of mind" is a classic warning about mechanical filtration. Though the aquarium water may be clear as crystal, alas, the brown gunk developing unseen on the mechanical filter medium remains part of the recirculating system until you "export" it by cleaning the filter. Often the ammonia given off by biofilm in a dirty mechanical filter can be an underestimated source of stubborn nitrate levels in your water. Other bacteria in the filter are constantly mineralizing organic phosphate. The resulting orthophosphate can be used by algae.
 
Sometimes you still hear that fish swim in their own toilet. Anyone whose wholesome drinking water comes from treated and filtered river water knows this can only partly be true. All detritus in the aquarium system, wherever it may lodge, is actually in the process of being transformed. It is decomposed partly by detritivores and more thoroughly by bacteria. In the sludgelike biofilm that forms in the mechanical filter, bacteria are already working to break down the organic particulates. The "biofiltration" process is an aerobic one, which takes up oxygen and releases ammonia, carbon dioxide and dissolved organic carbon molecules. Yet when you calculate the "bioload" of the aquarium, so often you estimate the mass of your fishes, but you don't take into consideration the high oxygen demand of that hard-working bacterial community in a dirty filter. You can sometimes reduce the biological demand placed upon oxygen reserves by the bacteria in your ostensibly "mechanical" filter if you're willing to tolerate Malayan Trumpet Snails in your filter, as I do.