I've got 20L and 25L tanks with bettas in, roughly 5 us galls. I use air driven box filters on both, I put the same ceramic rings I use in my external filter on the main tank with floss on top. I bought a decent pump which runs both of them but a small air pump would run one just fine. My water tests are always perfect. I also use them on my endler tanks which are 10 and 15 galls respectively. They hold quite a lot of filter media so work very well and are not too costly.