The canister filter has been in use for decades. Freshwater aquarists were the first to make use of them and they still do. Then along came the marine aquarium and the canister filter was employed to support those to. Canister filters have been around for so long that they are now very dependable and failures are rare.

Aquarium filtration techniques have progressed considerably in the last decade or so. Aquarists strive to make their reef aquariums dependant on natural filtration as far as possible. The use of 'live' rock is the major example. This rock can support the reef aquarium wonderfully, provided it is of sufficient quality and in sufficient quantity, and its capabilities are not abused by the aquarist. Live rock is probably the number one filtration method in use nowadays, in reef aquariums anyway. Another example of filtration is the DSB (deep sand bed) where natural methods provide support.

So what of the canister filter? With all these modern techniques perhaps the need for a canister filter is gone. Well, no it isn’t.

A canister filter can be used to house activated carbon, or filter pads or wool, to assist in keeping the seawater pollutant clear and dirt free. Some aquarists have put chunks (around ½" to 1? square) of live rock in the canister and report that the life within the canister is quite surprising - tube worms, tiny shrimps etc.

However, is the canister filter finished as far as a prime biological support function is concerned? No, it isn’t, is the answer (in my view). It is not the number one recommended marine biological filtration method nowadays, and rightly so. There are occasions though where the use of a canister filter is, what shall we say - convenient.

Live rock is very expensive, and there is little point in deciding to filter with it if sufficient quantity is not obtained. There are other decorative and marine suitable rocks that have no filtration capability, but are available at a far lower price. So the aquarist who finds the cost of live rock prohibitive is still able to consider a marine aquarium. Decorate with low cost rockwork and filter with a canister filter. There isn’t any need for a sump, another cost reduction.

I can nearly feel the hackles rising on some marine aquarists! But lets consider it further.

The low budget aquarist should, if finances allow, obtain two canister filters, though this is not absolutely necessary - canister filters are reliable as stated. One acts as a back up for the other. The compartments inside should contain coarse and fine filter material (the coarse before the fine). Following the filter material, and last in line, should come the biological media. This is easily obtainable and not too pricey. Sintered glass media is good, but there are others.

The canister filter is cleaned at least every four weeks, or as experience dictates. The coarse/fine filter material must be kept free flowing and the biological media protected from dirt. With two canisters, this can be achieved by cleaning one at two weeks, the other at four weeks, the first at six weeks etc. The biological media is not cleaned, just the coarse and fine filter materials. It goes without saying that the canister filters should have the correct capacity for the aquarium size they are to support.

Ah, I hear the question coming up! But canister filters are just nitrate factories aren’t they? Nitrate in excess is one thing the marine aquarium doesn’t need.

Up to a point that is correct. A canister filter is a nitrate factory. But the production of nitrate is not exclusive to the canister, it is produced by any biological filtration system. Another filtration system will produce the same amount of nitrate as the canister with the same amount of ammonia to start with. It is the nitrogen cycle, the natural breakdown of toxics. There is a difference however. The canister filter is having oxygenated water pumped through it, which means that the nitrogen cycle can only progress as far as the production of nitrate. Live rock, for example, will hopefully complete the full nitrogen cycle, breaking down the nitrate into gas that is released from the aquarium. Routine seawater changes are recommended for all aquariums. Where a canister filter is in use (for biological purposes) the seawater change must be completed as it is one of the methods for reducing nitrate. The aquarist using a canister can consider completing the nitrogen cycle by other means. A denitrator will remove nitrate efficiently, so that is an option. Sulphur denitrators are the best option (in my opinion).

Now things have become more complicated again. If denitrators are to be considered then, coupled with the cost of non-live decorative rock, and the canister filter(s), plus the bio media, the cost is rising. The option for a canister filter seems only sensible when a smallish aquarium is obtained (for example, seawater changes are more manageable and less costly). Larger aquariums seem better with live rock even though the overall cost is high.

This is not a recommendation for canister filters over other filtration methods. It is my opinion that live rock, coupled with a DSB in a sump, is a very good filtration option. However, for a smallish aquarium where water changes are not a problem and the bio-load is not particularly heavy, the canister filter remains a viable biological support possibility. The aquarist should, of course, cost the options very carefully, and use live rock where possible.

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