There are three categories of filtration: biological, chemical, and mechanical. Mechanical is generally the least often discussed, possibly because it's the most simple, and deemed less critical than the other types. However, mechanical filtration is an important part of the overall health and stability of your aquarium. Knowing the basics of mechanical filtration will help you avoid filtration problems in your aquarium, and understand how to deal with them if they should occur.
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What Mechanical Filtration Does
Mechanical filtration physically traps particles of uneaten food, fish waste, decayed plant materials, and other debris in the aquarium water. Mechanical filtration is the first stage of the filtration process, and should always be placed so that water coming from the tank hits this media first. Keep in mind that most mechanical media also promotes the colonization of beneficial bacteria, particularly in the case of sponges or pads. Although biological filtration is not the primary function of this media, cleaning or changing this media will impact the biological capability of the filter.
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Types of Mechanical Filtration Media
There are several types of filtration media, which are somewhat dependent on the type of filter they are used in. Some filters have pads, others have sponges, and some are packed with filter wool. In some cases, the mechanical filter media is fashioned to also hold the chemical filtration media, as in the case of cartridge media used in some power filters. These materials have openings that range from coarse to fine, thus allowing different degrees of filtration based on the pore size. Some filters will have multiple layers of mechanical media, with the most course being placed first in the water flow to remove large particles first. Ultrafine pore pads should be placed in the last position of the filter, to "polish" the water of tiny debris before the water is returned to the aquarium.
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Sponges, pads, or foam blocks are used in all canister filters and some power filters. They are sold in rectangles, squares, or circles that match the opening in the filter basket. Some may have several pore sizes available, which should be matched to the purpose for which they are used. Read the product information carefully to ensure you are getting the proper pore size, as very fine pore size will clog too quickly for first stage filtration. There are also filter sponges made to fit on a powerhead or filter intake to reduce the water flow as well as provide filtration. These are particularly helpful in grow-out tanks for small fry
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Filter wool or floss is used in a variety of filters. It originated in the days when we had the old style in the tank box filters, where it was stuffed into the filter box. It may not have been too attractive once it started collecting debris, but it worked. Although box filters are rarely used anymore, filter wool has continued to be used in canister and wet/dry sump or trickle filters. It is also used by some enthusiasts in interesting ways. For instance, those who do not care for carbon filtration will remove the carbon from a filter cartridge and stuff it will filter floss.Continue to 5 of 6 below.
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Often power filters will combine the mechanical and chemical media into a single cartridge. The exterior of the cartridge provides mechanical filtration, while the media inside provides chemical filtration. Some of these cartridges can be opened, thus allowing the internal chemical media to be replaced. Others do not open and must be discarded and replaced with a new cartridge once the chemical media inside is exhausted.
As a side note, it is a myth that carbon media can be recharged through boiling or baking in the oven. Carbon media should be discarded and replaced regularly. Some cartridges even go so far as to have finer mechanical media on the backside of the filter, to polish the water. Whenever using cartridge media, read the manufacturer's instructions carefully to become familiar with the proper way to mount and service the cartridge.
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Changing Mechanical Media
Over time, mechanical media will trap more and more particulate matter, slowly reducing the water flow through the filter. A common mistake made with filters is to never clean or change the media. If not maintained, eventually the mechanical media will become so clogged that the water flow will slow to a trickle or even stop altogether. If this happens, any biological colonies within the filter will die off fairly quickly, due to the lack of oxygenation. Another problem that occurs when filter media becomes full of waste is that the debris begins to decompose, releasing harmful toxins into the water. Such events can, and should, be avoided for the sake of maintaining a healthy biological balance in the aquarium. Mechanical media must be washed or replaced regularly.
To minimize the impact of cleaning on any biological colonies in mechanical media, partially fill a bucket with aquarium water. Then remove the mechanical media, quickly place it in the bucket of water, then gently squeeze and swish it through the water several times until most of the trapped particles have been dislodged. Place the media back in the filter and restart the filter promptly. This will eliminate most of the debris, while not killing off all the beneficial biological colonies within the media. Over time tiny particulate debris will build up in the media, and cannot be dislodged through cleaning. This will become apparent as the water flow, even after cleaning, will be noticeably reduced. In that case, it is necessary to replace the media. If the media is simply discolored, but still allows normal water flow through the filter, it is still functional and should not be replaced. Only replace it when water flow is reduced even after cleaning. Very fine pore media can only be replaced, and should not be washed and reused.
Because of the impact on the biological colonies, it's generally best to rotate the cleaning regimen in such a way that not all biologicals are disturbed at the same time. For instance, don't replace the filter media, at the same time as performing heavy substrate vacuuming.