Under gravel cleaners

[0sprey]

For a fish-only system, UGFs work just fine. But if you want corals or other inverts, cross them off the list. They become solid nitrate factories in a fairly short timespan.
Robert Fenner's Conscientious Marine Aquarist has an excellent overview of the different types of systems, and notes in detail the benefits and disadvantages of undergravel filters. Order it in from your local library and read it. Well worth your time.