A canister filter is an external filtration device used for holding various media which can provide chemical, biological and mechanical filtration. They have limited use for the majority of marine aquariums.
They are perfect for holding chemical filtration media such as activated carbon or phosphate removal resins. They provide good biological and mechanical filtration, but for the average tank this is not required. For most systems live rock and sand beds provide ample biological filtration. Canister filters are beneficial for a system that has limited substrate or rock (e.g. a hospital tank).
How They Work
Water enters the filter via an inlet tube that is submerged in the tank, typically passes down through the lid of the filter, then up from the bottom through the media that has been placed inside, and then pumped back by an internal pump to the tank via an outlet tube.
Those coming from the freshwater scene take sometime to even consider that something that is looked upon as a great filtration technique for freshwater systems, does not have a similar use and status in marine. Biological media such as noodles and bio balls that is often used in canister filters is an ideal place for aerobic bacteria to thrive. However this only provides aerobic biological filtration or the conversion of ammonia to nitrate with no chance for anaerobic biological filtration that can remove the nitrate from the system. The result is an accumulation of nitrate in the system. One method that is sometimes used to provide anaerobic biological filtration within a canister filter is to replace the noodles or bio balls with pieces of live rock. It is assumed that the interior pore spaces of the live rock are anoxic an provide an area for anaerobic bacteria to thrive.
Another situation to be aware of when using aerobic biological media in a canister is the consumption of oxygen that occurs. The conversion of ammonia to nitrate requires oxygen that is taken from the water with no means of replenishing it within the canister filter. Adequate gas exchange must be provided within the system. The use of a surface skimming devise is strongly recommended for canister filters used on marine systems.
Liverock inside canister filter to provide surface area for biological filtration.
Can I use the intake and return from the canister to feed my sump?
The intake of a canister is typically just a tube that goes over the side then down to the canister. If you connected that up, you will have a constant siphon from the tank to the sump that is no way regulated by the flow rate into the tank like a siphon box is. So it doesn't work, it is impossible to balance the flow rates.