I just put a Prism Deluxe skimmer on my 7-gallon tank on Sunday. It is already starting to pull some green stuff out of the tank. I was doubtful before I ordered it, but it actually looks like a pretty nice design for a combo unit. Definitely better than not having anything.
If you have any method at all though for running a submersible type, I have used the small Sea Life skimmers a like those alot. They really work great.
I am assuming you are looking for a HOT skimmer. There are some other HOT-type skimmers out there and you should have more options with a 20 than I have with a 7. Maybe someone can give you some firsthand experiences with the Bak-Pak units.
Good luck.