I'm not sure of all the finer points of a UGF, as I have never used one. Basically they filter the water through your gravel, which is where beneficial bacteria will grow. Without one, tanks are kept clean through an external filter. Regardless of what type of filter you decide on, you will need to do regular gravel vacuming to keep the tank clean and also remove nitrates from the tank.
Personally, i prefer external filters because I like sand in my tank. Also, when i had one in a betta tank, i found i had frequent mini cycles from over vacuming.
