Check out some of the reef sites, there are some DIY plans for various skimmers floating around. At a minimum, you could just have the current flow from the top of a tube, down to the bottom, and insert an airstone in the bottom. collection cup at the top, and bingo, cheap, easy, skimmer. Not that effective though. I've got a pretty big skimmer that used to be part of a central system at a pet store, I'll dig it up and see what it looks like.
