Filtration in the marine aquarium supports the number one requirement, and that is water quality. Whether the system is a fish only aquarium or reef aquarium, water quality is paramount. The next most important requirement is lighting for a reef aquarium.
The question here is a little too general. Are we talking of bio-filtration, or is it mechanical and chemical filtration? I don’t know what was behind the question so let’s have a look at them all.
Bio-filtration is the major filtration need in an aquarium, so let’s start with that. Bio-filtration is where the nitrogen cycle, or part of it, takes place. That is, bacteria take dangerous waste such as ammonia and nitrite and convert it. Without this natural service keeping a marine system of any type would be difficult, expensive and not so good for the inhabitants.
The major requirement for bio-filtration (at least the initial stages of the nitrogen cycle) is oxygen. So for anything to operate well this oxygen supply is required.
The first method with this in mind is the trickle tower, where water runs down with gravity through media. This is a good method of filtration, as oxygen is readily available from the atmosphere. The bacteria are not going to have any problems. The downside is that these filters produce nitrate and that point is the end of the nitrogen cycle.
So what about that old and well trusted method, the canister filter. These filters are very reliable and the modern ones are easier to service (this used to be a bind). The bacteria are completely submerged and depend on oxygen from the seawater, so it is important for the system designer to bear this in mind, that is the water surface should be open, and in addition if possible weirs should be incorporated, or a powerhead could be directed at the surface. Additional oxygenation may not be needed in a well designed system with correct water movement. Again, the end product is nitrate.
The under gravel filter is subject to the same oxygen considerations as the canister. However, the under gravel filter is, at least in my opinion, not a good choice because of eventual problems with media blockage. This filtration will not be considered further.
Though there are other bio-filtration methods the last one to be considered in this text is live rock. Live rock is natural in that it is nature’s product. The bacterial process in any filter type is natural of course, but live rock is natural overall. Live rock in sufficient quantity and of high quality can successfully filter an aquarium. A big plus is that this method can deal with nitrate if not abused – in other words, the filtration provides the full nitrogen cycle. That is a big plus. In addition, the very fact that it is rock means that the aquarist can use it to aquascape the aquarium, and it doesn’t matter if the system is fish only or reef. The bacteria again rely on the oxygen in the seawater, so the points noted about this are relevant.
It doesn’t take a detective to discover the champion bio-filtration system here, and yes, it is live rock. The downside is that live rock is expensive, and corners should not be cut to save money with this filtration. If money is a problem, then consideration to the canister and trickle methods can be given, remembering the point about nitrate.
Right, that’s that bit covered. What about mechanical/chemical filtration? There’s not much to be discussed here.
With this filtration the aquarist requires a method that is unobtrusive and reliable. There aren’t any oxygen considerations. The canister filter is the one, and ideal for the job. They usually have compartments where different media can go – sponge of various densities and other fine filter types to trap detritus, and carbon if used. As said, canisters are easier to service nowadays than they used to be and that is the main requirement – they should be regularly serviced and any carbon changed, and detritus trapping media cleaned or renewed. The cleaning can be under a tap, there isn’t any need for caution in respect of bacteria. (Note that this is mechanical/chemical filtration. If a canister is being used for bio-filtration then cleaning under a tap the media supplied for the bacteria is a no-no, as the bacteria will be killed with dire consequences.)
So there we have it, the best: live rock for bio- filtration and canisters for mechanical/chemical filtration. This is of course my opinion, but I would hazard a guess that there aren’t many who would disagree.