Matching your system's requirements to a particular skimmer usually involves reading the ads in magazines. Talking with other hobbyists that have or know of the item you are contemplating is always a good practice. If you live in an area that is "guppy-heaven", then finding other salt enthusiasts may be problematic. Our Boards, Chats & Clubs Resources page is a good place to start, and you can always post a message in on our About Saltwater Aquariums Bulletin Board or drop into Salt Talk Chat and discuss it.
Systems without a sump below the main tank are limited to the designs and models that are available. The Co-Current style is perfect for smaller tanks up to about 40 gallons or so. These are placed within the aquarium and are powered via a good air pump. They are hands-off and do a fair job. Here is a diagram of an In-Tank Co-Current Style protein skimmer.
