Here is information all about standard undergravel and reverse flow UGF setups, which are not ideal for use in reef tank systems because they can become nitrate factories. They are however suitable filters for fish-only saltwater aquariums, as long as this type of filtration setup is properly maintained.
From your About Guides, Stan and Debbie discuss the pro's and con's of using and undergravel filter for saltwater aquarium filtration.
From your About Host-Guide to Saltwater Aquariums, Don Carner discusses UGF's, how a they work, cost perspectives, and the pros and cons for using this type of filtration.
From your About Guides, Stan and Debbie Hauter give instructions and graphics for building your own UGF. You can build this sturdy, inexpensive Under Gravel Filter in just an hour, or so.