It really depends on the size of a tank that you have. Sponge filters are great for a small tank, but if you have something over about a 20 gal, you will need more than one. If you already have a HOB (hang on back) filter, you can try something like this...
http://www.amazon.com/AQUARIUM-TECH...3?ie=UTF8&s=home-garden&qid=1206319277&sr=8-3
You can cut a sponge designed for an aquaclear filter to fit over your intake to keep the moss out of hte filter.