Marine Aquarium Filtration Methods
Filtration is obviously essential. There are various schools of thought among marine aquarists, and many experts have extremely powerful and expensive filters which provide absolute cleanliness without the need for ‘biological’ or subgravel filters. For most of us, however, these filters are the most useful and cost-effective types, and provide good and consistent results.
The biological filter is exactly like that used in the freshwateraquarium, usually consisting of a perforated or slotted plate installed beneath the gravel, with a bubble airlift which produces a powerful current down into the gravel carrying debris to be broken down by bacteria. It is important that the airlift be as powerful as possible, to create an adequate water flow. There is a further very important function of this type of filter in the marine aquarium,the bacteria which develop in the gravel bed remove or detoxify the breakdown products of the fish. wastes, and so maintain the ‘buffer’ which is so important in stabilizing water conditions.
When a biological filter is used, at least 8cm (31/4in) of gravel must cover it, This is not ordinary builders’ gravel but usually coral sand, composed almost entirely of calcium carbonate and formed from the crushed skeletons of the coral animals. it may be very fine, or can be in coarsely crushed lumps of about 0.5cm (1/4in). Some aquarists use crushed cockle-shells, which are local and cheaper, and work just as well. All these materials are slightly porous, and contain cavities in which the bacteria lodge, so that the gravel bed can contain many more bacteria than it would if composed of non porous stone. The more bacteria, the more effec-tive will be their ‘buffering’ action.
All solid debris is removed, and the activated charcoal chemically breaks down some organic waste The only drawback to this orthodox type of filter is that large food particles may lodge in it and produce excessive amounts of waste as they decay. For this reason, power filters are usually usedin addition. The power filter is expensive, but pays for itself many times over. It usually consists of a large canister containing polyester filter medium together with activated charcoal. On top ofthe canister is a powerful electric pump. The filter draws water from the tank bottom through a plastic pipe and forces it throughthematerials which are not removed by biological filters — these are the substances which cause the water in an old-established tank to become slightly yellowish. In time, bacteria become established in this filter medium, making it doubly effective. The pump is very powerful; a size should be selected which is capable of shifting all the water in the aquarium at least three times every hour.
The output from the power filter is a powerful jet which is directed at an angle beneath the water surface, so that it encourages water circulation and dislodges food particles which may have lodged in the coral.
Because the water leaving the pump is clean, it can be used in conjunction with a biological filter to prevent large particles of debris lodging in the filter bed of gravel. This is the process called`back-filtration’, where the water from the power filter is pumpeddown under the gravel and emerges into the tank through the gravel surface. This technique avoids the packing-down of gravel which occurs when a biological filter has been in use for a long while.
Other types of filter can be used, such as corner filters or outside types. The normal outside filters are not very practical, unless they have a tightly fitting cover, as the constant bubbling will otherwise cause rapid water evaporation, so that the filter and any items near it soon become encrusted with salt.
In addition to filters, aeration stones are usually fitted. It is scarcely possible to produce too high an oxygen level for reef fish, which also enjoy the constantly moving water.