I used to use the Hagen Elite. They came in single sponge (good for 10 gallon and 5 gallon) and dual sponge (I have one of these in my 20g, 29g, 40g tanks). I believe these are no longer available; last year I couldn't find them anywhere locally or online, so I ordered a couple of less expensive but identically-designed filters from Amazon, and after a year they are doing the same good job as the Hagen. The sponges seem to be a bit better made too, heavier so they may last years. I'll attach some photos of the Hagen and the one I got last year. They attach to a small air pump with tubing. I like this style because in a back corner they take up little space, and a chunk of standing wood or plant in front of them hides them. The angle of the sponges is adjustable.
If there are two or more tanks in the same area, you can use a larger air pump and have one of these filters in each tank. This is very economical (electricity), very easy to rinse under the tap at each water change, and extremely good mechanical and biological filtration. My sponge filter tanks are always crystal clear.