I don’t know if Aquael PATmini is availible in Your country, but in my opinion it is a good filter for small aquarium. Highly adjustable flow rate is nice, and the filter itself is very,very small. In my opinion using air powered filter is good, when You have many tanks on a shelf, since the power input will be small as opposed to electric turbines, but the air pumps are noisy, even when producer says they should be silent.
I would propose two Aquael filters:
FAN MIKRO Plus – [X}
PAT MINI – [X]
In Poland PAT MINI costs about 6 Euros.