What is a canister filter?
A canister filter is a plastic unit which holds the filter media with a motor unit attached on top. Canister filters needs to be under the tank water level as it is a gravity fed system with a return pump. If you buy a proper (read more expensive) brand of canister filter you will only have to clean the filter once every 3 months, some of the top units come with an indicator which alerts the user when it is time to clean it.
Some canister filters comes with a heating element built into the motor housing to heat the tank to a constant temperature.
These filter units are great for chemically altering your water quality as they allow a large volume of filter media to be used. In freshwater systems where you want an acidic PH you can run peat fibre or granules (not advisable in the Western Cape), in a Malawi tank you can use coral chip/shells. For marine systems they make great phosphate and carbon reactors.
As far as filtration goes the problem with canisters in a marine tank is that they reduce waste as far as Nitrate, however unlike a Deep Sand Bed they do not turn Nitrate into nitrogen gas.
They can be used as a water polisher by filling the canister with filter floss and "vacuuming" rocks etc which will remove small particles. If this is to be done then it should only be run as long as you are "vacuuming" the tank. Thereafter the floss must be discarded and the unit washed and stored.
When used as a carbon and phosphate reactor it is important to understand that the purpose is no longer to remove particles from the water but rather have the water flow through the media.
Carbon is used to remove toxins and polish water. It can, in the place of any other filter media, be used in a canister filter.
As with carbon, Phosphate media can be placed in a canister in the place of traditional filter media such as ceramic rings, sponges etc Phosphate media such as RowaPhos, PhosBan etc. remove phosphates in the water that will lead to algae growth.