I have a 39 gallon, a 30 gallon and a 20 gallon all with emperor filters, and they are great! When I first filled my tank that I bought from someone else, of course there was a bunch of stuff in the gravel, even after I washed it, and it was very cloudy. But about 10 minutes after plugging in the Emperor, it was crystal clear. They really do good and have a nice high water output. I believe they'll handle up to a 55 gallon, so in a 20 you'll have super filtration.