“A reef aquarium must be provided with a good mechanical filtering” some say, “a reef aquarium works better without a mechanical filter” is others’ opinion. The strange thing is that both the statements are true. Every reef aquarium should have the opportunity to be filtered mechanically, but there is a good chance that it will no longer be necessary to use this procedure once the tank is well “run-in”. At the beginning, some annoying algal coverings often form, and the pieces of floating algae should be removed from the water with a filter in order to obtain an environment of good quality for the invertebrates. In the long term, however, a mechanical removal represents for all the filtering invertebrates a remarkable competition, since their existing chances depend on what the filter removes. A use of the mechanical filtering should only be made when the organic charge of the suspended substances is so significant that it negatively influences the corals and the other invertebrates.
What is the best method?
Obviously, in any filtering system the water passes through some holding means, yet it is possible to distinguish three methods of mechanical filtering.
Filtering by aspiration
The most used technique in the marine aquariology is that of the mechanical aspiration. This procedure, basically, consists in using a container filled with the holding means (often synthetic wool, sponges, etc.) and placed at the aspiration opening of the pump, in such a way to force the water to pass through it.
Advantages: the circulation generated by the pump remains largely available, at least as long as the filtering material does not become blocked.
Disadvantages: the filtering substratum represents an obstacle: between it and the pump a decrease in pressure is generated which degasifies the water. The consequence is that in some cases from the exit opening of the pump some air bubbles come out. The greater the obstacle to the water flow will be, (for example due to a dirty filter or a too powerful pump), the huger the quantity of produced bubbles. In the case of pumps with an excessive yield, part of the dirt held by the filter can permeate and be put back into circulation.
Filtering by pressure
This method is relatively little used in the reef aquariology for the cleaning of the water. Usually, this procedure finds an employment in the diatom filters, with which the water is compressed through a very fine powder obtained from the shells of some algae (diatoms). This type of filtering, however, is not applicable for a continuous employment, but is only used for short periods. In theory, a filtering by pressure would be possible applying a filtering container to the exit nozzle of a pump.
Advantages: the water of the aquarium is not subjected to any decrease in pressure and therefore does not suffer the degasification.
Disadvantages: the yield of the pump is almost completely annulled.
Very frequently, in the reef aquariology this technique finds application. It results as rather simple to realise if one disposes of a filtering tank placed under the aquarium, because in this case it is obtained almost “for free”: the falling water that comes from a an overflow of the aquarium, is conducted to a pierced container filled with the filtering means (e.g. synthetic wool), passing through it with the sole help of the force of the earth attraction. After the passage through the filter, it ends in the collection tank from where it is pumped again towards the aquarium.
Advantages: decreases in circulation due to the filtering means never occur, as well as the water degasification, and it does not become necessary to use an additional pump because the filter is managed by the pump that pumps again the water upwards in direction of the aquarium.
Disadvantages: this technique requires a collection tank that due to the falling water may cause some noise.
An answer to the initial question, therefore, depends on the system used in the aquarium. If you design an aquarium with a filtering tank, filtering by gravity is the best choice. If your aquarium does not have a collection tank, then filtering by aspiration results as the easiest method, because of its easy realisation. However, this filtering method is not as effective as the gravitational one. Anyway it is important for any system to regularly clean the filtering means, because only in this way can the procedure be completed. As long as there is organic material in the filtering material, the latter will be still present in the water cycle and if it is not removed after a short time it will decompose, polluting. For this reason, what decides the quality of your mechanical filtering is not the method, but the frequency of the cleaning: the more often you will clean the filter, the better its functionality will be.