Talk about filters for small aquariums, conducted in the magazine, is very relevant for many aquarists. Especially beginners. Most publications about Fish Tank Filters are focused on the capacity of about 100 liters or more. It is believed that it is from 100 liters that a “serious” aquarism begins. But the world produces and sells a huge number of aquariums of 60, 40 and even 20 liters. The popularity of small aquariums is especially characteristic of our country, where, for various reasons, not everyone can afford a large-sized reservoir.
So, let’s define – what kind of aquarium can be considered small? I propose to take the “small” volume up to seventy liters.
The meaning of filtration in general is reduced to two processes: mechanical filtration (increase in water transparency) and biological (improvement of its chemical quality). In an aquarium with a well-set filtration cycle, you can place more fish and other aquatic animals. Do not, of course, forget about aeration. Given the shape of most commercial small aquariums with a free water surface insufficient for natural diffusion, as well as overpopulation, very often observed by amateurs, aeration in such an aquarium is highly desirable. Biological filtration occurs due to the activity of bacteria on the filter material. From this it follows that the volume and total surface area of this material should be maximized. In this case, the natural limitation is the overall dimensions of the filter.
The next filter I used was a little more “solid” “FAN-mini” of the same AquaEL. His test is described in the number 3 of the journal for this year, so I will not repeat. I note only that this sample has a slightly larger volume of filter sponge. An interesting design feature is the intake of air directly along the axis of the pump impeller, and as a result, the work of the ejector is stable, unlike other filters. However, I was faced with the increased noise of this filter, which really interfered with sleep at night. Perhaps this was due to poor balancing of the rotor.
I did not manage to find anything else for aquariums of about 20 liters in volume. So how do you equip a 15-20-liter mini-aquarium? The way out is simple: to use widely known in the recent past, and now – half-forgotten airlift filters (see photo). Such a filter, made independently or bought for very modest money (no more than 50 rubles), is driven by a compressor. Changing the air flow from the compressor, we change its performance. Air lift filter does not form a powerful jet, which may be undesirable for aquatic plants, has greater safety in use than electric pumps, since there are no electrical parts immersed in water. Another advantage is the aeration simultaneously with filtration. Its undoubted advantage can be considered the fact that its dimensions (in the case of its independent production) are determined by the aquarist himself. So, how to make it? The basis of the filter (see the figure) is a tube with a lot of holes on the side , it is wrapped with filtering material  from the outside, and a hose from the pump  is inserted into the tube. All this is bound together, for example, with copper wire [4, 6]. To keep the filter in the submerged position, you can use sinkers  of heavy but safe metal in the aquarium. The resulting structure is installed inside the aquarium at some distance from the surface. That’s all.
The lack of an air-lift filter can be considered the noise from air bubbles, which with a large flow rate can be significant.
Unlike simple aeration through a spray gun, inside an air-lift filter, small bubbles form into larger ones that burst on the surface with noise. Another disadvantage of this filter is its relatively low performance. Thus, the use of air-lift filters is limited to small aquariums. Especially I would point out the advantages of using them for a children’s aquarium: they are ease of use, safety of use and low cost.