For Mechanical Filtration
An aquarist with an undergravel filter can add a hang-on-tank canister filter to their system to remove free floating particulate matter from the water that would normally get drawn down into and trapped in the substrate. By continually running a canister filter on this type of aquarium set up, it contributes to improved water quality.
An aquarist that has a semi-reef system with fish and some live rock in it can choose to install a canister filter inline to act as a "pre-filter" to remove unwanted waste, particulates and detritus from their tank water before it passes into or through their biological filter (i.e. wet/dry trickle) or main tank water supply, such as into a sump. This can also be done on full reef tanks with live rock and corals in them, but the debate about continually running mechanical filtration in this type of system is that such a set up filters out beneficial plankton life in the water that many marine organisms feed on.
Hang-on-tank canister filters are very often only used as a means of mechanical filtration during regular tank cleaning and maintenance care routines. They are also one of the simpliest ways to control heavy copepod and amphipod larval blooms. You know, those little white bugs often seen swimming or crawling around in your aquarium.
For Chemical Filtration
For an aquarist that needs some help in clearing up a water quality problem, they can place granular activated carbon (GAC) in the media chamber to help eliminate odors, medications or other contaminates in the water, as well as use other types of absorbing products (read reviews & compare prices) that are designed to remove nitrates, phosphates, silicates and other unwanted chemical elements or compounds. This type of filtration also applies to the filtering of fresh tap water prior to using it to make-up sea salt mixes or adding it to an aquarium as top-off water.
For Biological Filtration
Even though many canister filters are designed for this purpose and a lot of aquarists use them in this way, in our opinion they are not a good choice as a "main" source for biological filtration. They may be o.k. for smaller systems, but most do not have a chamber big enough to hold a sufficient amount of bio-media in them for larger ones. Therefore, they are inadequate to use solely for this reason, but one can still be run in conjunction with another form of biological filter, such as with live rock or a wet/dry trickle filter, for additional mechanical filtration of the aquarium water.
Now, one of the most important things to consider when purchasing a canister filter is the water flow rate, NOT based on what the manufacturer says you will get, but what you will ACTUALLY get after taking a few other factors into consideration.
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