I am running an Ecco 2234 on my 37 gal
and it is woefully inadequate, with particles floating all over and the intake is not pulling them in, so it would probably work on a smaller tank. I'm going to upgrade to the 2236.
I don't think the Ecco series is as powerful as the classic series, like the 2213, but they are so much easier to maintain. I think a 2213 would be fabulous on a 20gal. I saw on the Eheim site that they now make media baskets for the classic series, so you can pull them out and arrange them, which would change my life when I go to clean my 2215. Something to think about.