I have no idea how you would put a skimmer under your tank without a sump. Unless you mean just having a tank underneath and putting your skimmer in it? If that's what you're talking about, its basically the same thing as a sump, just without dividers. If you just don't want a tank under your display tank at all, you can go with a HOB (hang on back) skimmer, which hangs on the side of your display tank.