I want to get a new filter for my 20 gallon. I use the aqueon quiet flow 30 but it has not been working great lately. Is the AQQA aquarium sponge filter a good filter, if not any recommendations for the best filter?
If you are referring to the powered AQQA sponge (like this image from my crowded skittles tank), then yes, I think they work pretty well. I have a couple of the small 3w ones, but for 20's and 29's I use the 5w one. They are the only filtration I have in the tanks and I really like them for shrimp and nano/planted setups.
The Aqueon Quiet Flow filters are great filters....until they aren't. Their biggest flaw is the impeller bushings that have a very bad habit of falling off if you to clean the impeller. You can purchase replacement bushing, but they are not easy to find.
The Aquaclear design has been proven over time. I had one that lasted me over 20 years. Either an Aquaclear 50 or 70 would work for a 20 gallon tank. The AC50 is rated for 20-50 gallon tanks, but is great for 20-40 gallon tanks. . The AC70 is rated for 40-70 gallon tanks, but I would say it is more suitable for 29-55 gallon tanks.
There are a couple of newer filter designs that look pretty good, but I have not tried them. One is the Seachem Tidal filter designed by Sicce that looks good. For a 20 gallon tank, I would chose the Tidal 55 at 200 gph. Another option is the Fluval C series filters. Fluval and Aquaclear are part of the same company (Hagen), so the C series shares some design features with the tried and true Aquaclear. Either the C3 or C4 would work with a 20 gallon tank.