Keep in mind that these skimmers have a somewhat large foot print. AquaC skimmer works for me, but not with the pumps that come standard and their biggest advantage is small footprint same goes with Tunze. Need to oversize the pump and it will work unless there is above average bioload. If your tank has heavy bioload then a larger skimmer mentioned above will work better. With a 20 gal sump there is not a whole lot of room to play with especially if you have refugium. It all depends what you want to have in that sump, or you can always upgrade to a larger sump and then you don't have to worry about a foot print limitations.