If you haven't spent your hard earned cash on the seaclown yet, let the masses try to disuade you. There are MUCH better skimmers, even in the lower price range you would do better with.
I have a seaclone that came with the setup I bought. It's an older model & since I've done the mods (cut down the innermost tube, cut down the collection cup inner tube, added an air flow regulator, reduced intake tube so powerhead is closer to surface; all free mods) it produces a very dark skimmate & is adequate for my needs. I think it would suffice on a 20g. Mine is NOT for sale but that doesn't mean you have to rule that model out.