What is a canister filter/external filter?
A canister filter/external filter (let’s just call them canisters for clarity) is usually a plastic drum or container with a motor on the top of it. It is usually filled with various media via plastic baskets and sucks water, usually containing excessive nutrients, from the aquarium via a flexible pipe into the drum or container where the water is forced through various media (discussed later in the article) and returned to the aquarium after being cleaned, via the motor at the top. There are various types; usually it is a sealed system and a wide range is available from all aquarium stores. Typical makes would be Eheim, Fluval, Tetra and many others are available as well.
What advantages are there, if any, in using one?
There are 4 advantages of using a canister filter in aquaria in my personal experience:
(1) for biological filtration
(2) for mechanical filtration
(3) for chemical filtration
(4) for additional water volume and stability
For removing waste from the water system and converting it from ammonia to nitrogen gas via a bacteria colony. This is the basis of the nitrogen cycle and a very important filtration system. Life in aquaria would not usually exist without this filtration.
For removing large particle waste from the aquarium usually by the use of sponges or in some cases brushes, this then allows the biological filtration to be much more efficient with the smaller particles of waste.
For removing medicines, organic waste and toxins from the water stream.
Additional water volume
This is usually an advantage for smaller aquaria such as Nano cubes and systems under 20 gallons if sump less. This also has the advantage of temperature and salinity stability due to the extra water in the canister. The canister also adds flow to the aquarium as well when the cleaned water is returned to the tank via the motor which moves the water.
What disadvantages are there with using one?
Well, most seasoned reef aquarists have discovered that canister filters can get dirty. This can, and usually does, eventually lead to the canister adding nitrates to the system. Basically detritus and other fish waste plus uneaten food or dead organisms gets trapped in the mechanical stage of the canister filter and, because the bacteria in the biological filter doesn’t get chance to get at it, the waste won’t be converted via the bacteria into harmless nitrogen gas. Usually the waste is just trapped in sponges or falls to the base of the canister and becomes a serious problem if left unattended. However, if cleaned regularly, this can be avoided.
What media is available?
There are various media available I wouldn’t be able to list them all, however a few examples would be:
Ceramic rings, bio balls, live rock or other rubble type stone.
Sponges or brushes.
Carbon, Rowaphos or Phosban, and many other similar type media which can remove chemicals from the water stream.
Is there any point using a canister with little or no media in it?
Yes there is, I think that within the reef aquarium a canister can be very useful for usage with carbon and a phosphate media, plus as previously mentioned it is very useful for additional water volume on the smallest aquarium and water systems.
What size does it need to be?
There are various sizes of canister filters available with various flow rates and manufacturers do list their preferred models for particular aquarium sizes.
However, I don’t think it is accurate to estimate or suggest a particular size as it all depends on what media is being used in the canister and what you intend to use the canister for. As you have already discovered they can be used in several ways so size will depend on intended use
How often does it need to be cleaned?
Again this depends on its use within the particular system. If a canister is to be used for reef aquaria, then I’d suggest weekly if mechanical filtration is used, monthly if chemical filtration is used and every 2-3 months or longer if biological filtration is used. As you can see this is where the problem begins and why most seasoned reef aquarists don’t like them and suggest not using them. If all 3 media types are to be used then shutting the canister down every week is and will become a pain as the media would need to be removed to get at the sponges to clean them, not practicable and pointless as well. With my experience with canisters which is around 25 years now on all types of aquaria I suggest that they are useful for chemical filtration and for adding water volume to smaller systems. I currently have a 92 gallon corner aquarium with a 13.5 gallon sump and do not use a canister filter at the moment on this system, but would consider doing so in the future if I felt I need one for my individual needs.
Well, what have we learned with this article. Basically that a canister filter can be useful and of benefit for the smallest of aquaria, and for hospital tanks to remove the medicines from them and sump-less systems because of the added water volume, but we have also learned that generally they can be a nuisance for most reefs because of the constant cleaning and shutting down to change and clean media. We have also learned that there are so many different types of media available that it is generally down to the individual aquarist whether it is practical and useful and the usual website gossip, hearsay and jumping on the bandwagon has some merit but is also not always the case. It’s up to you whether you use one, and only you will know if it’s a useful tool or a hindrance for your reef aquarium.