Planted tank

Mechanical filtration in aquarium

Mechanical filtration in aquarium is the process by which particles of matter floating in the water are removed. In other words, it is the process of separating the solid phase from the liquid phase as it flows through a porous medium. The water flowing through the filter is also purified of dissolved substances. Mechanical filtration is a very important process in an aquarium – thanks to it, we can enjoy clear water in the tank.

Filter cartridges

The size of the removed particles depends on the filter materials used, the choice of which is very large on the zoological market. There are sponges of different densities, perlon wool, fleece and ceramic materials. During the use of all these media, a process of collimation (clogging) occurs. This results in reduced flow, as well as the retention of increasingly fine particles. The filter, or rather the filter cartridge, can become completely clogged, preventing water flow. As you can guess, colmatization occurs faster when using dense filter materials as well as when caring for very dirty fish species. An example of “filthy” fish are (wood-eating) Loricariidae or goldfish.

Physical treatment of water

The physical processes that occur during water filtration through porous materials have been well understood and described by mathematical formulas as they are used on an industrial scale in wastewater treatment and water treatment plants. Of course, I won’t go that deep into the subject. However, it is worth looking at the variety of physical processes that affect the purity of aquarium water.

Sieving of suspensions

The filter cartridge behaves like a sieve, trapping particles larger than the bed pores on its surface. This shows up especially nicely on the sponge and fleece. We use this process to a large extent by placing the so called prefilter on the pipe sucking water from aquarium to the bucket filter, i.e. simply a thick sponge that stops large impurities and protects the main filter bed against fast clogging.

Sedimentation of suspended solids in the pores of the filter catridge

It refers to particles that have not been retained in the sieving process. The bed pores can be thought of as winding channels of varying diameter. Every so often, water goes from narrow to wide channels where the flow velocity decreases. This results in sedimentation (deposition) of particles. As these pores are filled, the flow velocity in them increases and thus sedimentation decreases. The role of the deeper wide tubules is then increased. In this way, successive layers of the bed are gradually filled until the quality of the water flowing out of the filter deteriorates.

Stopping the finest particles

The finest suspended particles, for which the filter cartridge channels are too large and which have too little density to be retained in the bed by sedimentation, can be bound by the electrostatic forces of the bed.


In the pores of the filter cartridge particles can aggregate (join), which increases their size. Flocculation increases the probability of their retention in the filter bed.

Filter material selection

Mechanical filtration in an aquarium will run smoothly when filter cartridges are selected accordingly. In multi-chamber internal filters, manufacturers usually provide a set of filter materials. They are properly sized and perform well in the average aquarium. However, if you have a need to experiment, you can dispense with a module consisting of several chambers (which, for example, constantly gets clogged) in favor of a sponge. If the fish are very dirty, it is worth installing a sponge with higher porosity. It won’t catch the finest suspension at first, but over time, as its pores fill, it will. Most importantly, however, it won’t get clogged after a few days of action. The type of filter cartridge can also be modified in external filters. If the bucket is to be a mechanical filter then it does not need ceramic for biological filtration – it can be replaced with a sponge for example.

Mechanical filter cleaning

An important rule in using mechanical filters is their periodical cleaning, and the frequency depends on the aquarium inhabitants. When caring for dirty fish species you need to do it more often, even once a week. Rinsing out the mechanical filter not only improves the water flow, but above all allows to get rid of a considerable organic matter load from the aquarium. If only one filter (usually a bucket filter) is used in an aquarium, it is not advisable to open and rinse it frequently as it also plays the role of a biological filter. The use of the above-mentioned prefilter in such a case makes the operation of the device much easier.

More problematic are small tanks with small internal filters that act as both a mechanical and biological filter. Here, too thorough cleaning of the cartridges will always result in an unbalanced biological equilibrium in the aquarium. Therefore, internal filters that have two or three chambers are a good option. This helps us avoid cleaning the entire filter cartridge at one time. Several chambers also allow the use of smaller ceramic rings as a cartridge, which will ensure good biological filtration.

dr inż. (Ph.D.Eng) Aleksandra Kwaśniak-Płacheta

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