How a cannister filter works.
The water is gravity feed from the tank into the filter (a chamber that can be any shape but is usually cylindrical, that chamber can be divided into small er sections where different types of media are used to achieve different types of filtration) then it passes through two to three chambers depending on the size of the filter and is then pumped back into the tank by an internal pump that resides at the top of the filter.
On small cannister filters where only two chambers exists one would contain some foam to filter the fish waste and other water based particles. With the larger units where there are three chambers two are used for filtering the water with one containing a coarse foam to collect the larger particles and the second to collect the finer ones, the third chamber hold the bio-media.This is the media that bacteria grow on to keep your tanks eco-system running smoothly, the bio-media for cannister filters are generally made from ceramic and look at lot like a large tube noodle. the media is about 1cm long and 6mm in diameter with a hole in the middle, this design give the maximum surface area for the bacteria to grow on. It does take some time for the bacteria to grow so if you are starting with a new tank you should wait a few weeks before adding fish to it, then add a few fish every couple of days to give the filter time to adjust to the extra load that is being placed on it.
filter media achieve cleaner water through three basic types of action.
Chemical, physical and biological.
Chemical - the water chemistry is altered by a chemical process such as the actions of ion exchange resins, activated carbon or a foam fraction unit.
Physical or mechanical filtration is simply a sieve of some sort removing physical particles from the water. A good way to demonstrate the physical action is to put a teaspoon of fine beach sand and a teaspoon of sugar in a warm cup of water. Stir it for a few minutes then tip the lot through a handkerchief. The sand will be recovered but not the sugar. The sand has
been removed by mechanical filtration. You will have to come up with something elapse to remove the sugar because it is now a dissolved compound and that will require either a biological or chemical solution.(Plants are also bio filters).
If you tank is established you should still be careful and not add to many fish at once or you may find that the filter hasn't had time to adjust and this could cause problem like an excess of ammonia(fish waste which hasn't been killed by the bacteria in the filter) which can be fatal to fish.
There are many types of cannister filters on the market these days and below are just a few of these.