The Canister Filter – A Thing Of The Past Or Still Useful?

Technology marches on and there are quite a few devices that are available for use with a marine system. The canister filter is not new and has been with us for a considerable period.

Canister filters are made by quite a few manufacturers. They are usually shaped, err, like a canister, though there are models available that are square. As time has passed they have become very reliable.

A canister filter could be considered a closed circuit. Seawater is delivered to the unit down a flexible tube, passes through the canister media and is then returned to the aquarium through another flexible tube powered by a built-in pump. It is best if the intake and outlet are at opposite ends of the aquarium, though perhaps this is not so important nowadays when the high flow rates within the aquarium are considered. Nevertheless, it is best to keep the intake and outlet apart.

What can the canister be used for? Biological filtration is the first use that comes to mind. Though filtration this way has been superseded by live rock an aquarium can still be successful using a canister, and many aquarists still do. The cost of live rock is high, and so canisters find a use. They need to be filled with a good bio-media, and this media needs to be protected by efficient mechanical filtration, such as sponges and filter wool. The cleaner the bio-media is kept, that is, kept clear of sediment, the better. The canister needs to be sized to the size of the aquarium, and the manufacturer’s specifications will include recommendations. One point about these recommendations – the flow rate advised by some manufacturers is often with the canister empty, when it is filled with media it will be less. Consideration can be given to using two smaller canisters so there is a backup. The canisters can be serviced alternately.

Cleaning the mechanical media in the bio-filter is important otherwise the seawater flow will decrease. It is a good idea to clean this media after two weeks. It may be that it remains reasonably clean in which case increase the period to three weeks and so forth. Sometimes it is recommended that the filter should be cleaned when there is a noticeable reduction in the flow rate. This is not really acceptable because for this to happen the media will be very dirty. By cleaning and checking the condition of the mechanical media the aquarist can arrive at a reasonable service period. A watch should always be kept on flow rates even with more regular cleaning because of the importance of bio-filtration – a failure or reduction could result in serious consequences. In addition, the bio-media itself must never be washed in tap water as this will kill the bacteria. If cleaning is essential then seawater should be used, and this can be at a routine change using the old seawater. The media should be very gently stirred with the fingers under the seawater to remove dust and debris. It will never look ‘as new’ but this doesn’t matter.

If the filter is used for bio purposes then the seawater leaving the filter will be low in oxygen. This is because the bacteria that carry out the bio-filtration function are oxygen hungry. The oxygen can easily be replenished by releasing the returning seawater at the surface.

There is an important point with bio-filtration in a canister filter. This is that the ‘Nitrogen Cycle’ will arrive at nitrate and stop. The ‘Nitrogen Cycle’ is the process where bacteria convert ammonia to nitrite, then the nitrite to nitrate. A watch should be kept on nitrate levels, though this should be done in any marine system anyway.

If other means of bio-filtration are in use then the canister is not redundant. It can be used for purely mechanical purposes, that is trapping detritus that is in the seawater. In this case only mechanical filtration such as sponge and filter wool are in use, and these can be washed under the tap. If they are showing any deterioration they should be replaced. This of course also applies to the mechanical filtration within the bio-filter.

The canister can also be used for activated carbon and/or anti-phosphate media. These are put in after the mechanical filtration. Certain anti-phosphate media types with a very fine particle size are usually better in a fluidized reactor, as in a canister they could tend to clog. Cleaning the mechanical filtration area is the same as above, and a note should be made of when the activated carbon and/or anti-phosphate media are due for renewal.

There is another use for the canister. If a small canister set up as a bio-filter is kept running on the display aquarium, or better the sump as connection is easier and the intakes/outlets will not be seen, then a permanently ready bio-filtration unit will be at hand. This is useful for those aquarists who have wisely used a small aquarium for quarantine purposes. This could be required again for quarantine of a new arrival, or as a hospital area for a fish with a problem. There isn’t a need to keep this aquarium full of seawater, but if required it can be filled and the bio-filter is ready.

So the canister filter is not out of date. Modern types are very reliable and much easier to service than the older models, which could be fiddly. Having a canister in the system could certainly be useful.

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  1. I’m using a Fluval canister filter on my nano. It’s running foam (for mechanical filtration), carbon (for water polishing), GFO (for phosphate removal) and Purigen (for removing dissolved organics). I *must* clean it out monthly or nitrates skyrocket and LPS show clear signs of distress.

    On a small, skimmerless tank a canister filter works just fine IME. However, it doesn’t replace good husbandry — quite the opposite, without weekly water changes and a monthly clean-out the canister filter can be a source of real problems.

  2. Hi Andy.

    You’re certainly getting your money out of your canister.

    You’ve found your cleaning need, which is good. I clean mine every four weeks too, a coincidence. The need does vary though for diffent systems and uses.

    I heartily agree with your last paragraph – good husbandy!

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