Canister filters have been around for decades. They’re usually shaped like a canister (strange that) but can also be square. The media sits in the canister under the ’lid‘, and it is in or on the lid that an electric motor is placed. Water to the filter arrives via a flexible tube, and is returned by the same method. Canister filters were used, and still are, by freshwater aquarists, and have been adopted up to a point by the marine hobby.
Canister filters are very reliable though some makes, as in most things, are better than others. There are a couple of points that need mentioning. The first concerns flow rates. It is the habit of many manufacturers to quote flow rates for an empty filter. Without the media in position the flow rate will be higher, so in the real world expect the flow rate to be perhaps up to a quarter or so less. Flow rates are additionally slowed by the lift – if a filter is placed on a level with the aquarium there is little or no lift, if it is below the aquarium the electric pump has to lift water from the filter to the highest point before entry to the aquarium. It is important to know this distance and check the filter performance from manufacturer’s data.
Ok, back to the media. The question could be taken three ways. What is the best media to use in relation to a marine aquarium, the best for bio-filtration, and the best for mechanical filtration.
As far as marine aquariums are concerned, the bio-filter media of choice is good quality live rock, in my opinion. Therefore the canister filter, using this as a “rule“, would be redundant as a bio-filter. There aren’t any rules, however, and though the canister filter is not the best bio-filtration method to use overall it is not redundant in this respect. Nevertheless, if live rock is to be employed, the canister filter is demoted to mechanical filtration.
If the canister is to be used for bio-filtration, then some care on the media set-up is needed. Bio-filtration is the ‘life support’ for the livestock and obviously is very important. Though it may be against some manufacturer’s suggestions, I have found that having all the mechanical filtration in front of the bio media is advantageous. I found a fine filter, sandwiched between two medium grade sponge filters, does an excellent job on protecting the bio media from debris. It is very important that the mechanical filtration is cleaned at regular intervals (the period dependant on the cleanliness of the water, perhaps every two to four weeks). In addition to helping to keeping the bio media clean, it maintains water flow. The bio media available is varied, however sintered glass, which is in the form of small tubes, seems very good. As there are lots of channels within the glass, it can act somewhat as does live rock for a while. In other words, nitrate can be reduced by bacteria. This benefit is usually not long lived however, as fine debris does inevitably find its way onto the media causing some clogging and the anti-nitrate advantage is lost sooner or later. (This media should not be used intentionally for any anti-nitrate properties.) The media still remains very good for ammonia/nitrite filtration. The media, if cleaning is required, should be gently cleaned in warm aquarium water, never in tap water. The latter will wipe out the bacteria. A final point to note when bio-filtration with a canister filter is employed, is that the unit will usually produce nitrate (perhaps not straight away as indicated). When nitrite has been converted to nitrate, the nitrogen cycle stops. If there isn’t a low oxygen environment for bacteria to inhabit, then nitrate will remain. A check needs to be kept on the level.
If the filter is to be used for mechanical purposes, then the same mechanical filter media as above can be employed. This media can begin with medium grade sponges, perhaps two layers, and end with fine grade filters. Many manufacturers provide ready made filter materials such as sponges so the fit in the filter is good. Manufacturers usually give suggestions for mechanical filtering. In addition, activated carbon can be used in the filter. It is usually placed after some mechanical media. Activated carbon is not always a necessity, and should not be used continuously if there is a need but periodically. The media removes trace elements from the water. When exhausted (manufacturers will suggest life expectancy) the media should be discarded, if more is required it should be new. Similarly, anti-phosphate media can be used. This can be used continuously if necessary, changing the media at the manufacturers recommended periods. Some anti-phosphate media has a tendency to clog and/or channel, making it far less useful. The types that do this are usually the ones with very fine media.
So the canister filter is useful to the marine aquarist. It is not ‘number one’ for bio-filtration, but can still be used as such, with success provided the aquarist maintains it properly. The filter is also useful for assisting in the removal of debris from the seawater etc. Some aquarists, having a canister filter available and not requiring filtration from it, have experimented with other media, such as small pieces of live rock debris and very coarse sand. I don’t know the results of these experiments, but I do know that in my canister filters using standard media, when doing maintenance I always have to remove very small shrimps and snails and return them to the aquarium.