The principle of the UGF is very simple: You want to suspend the filter material (sand/gravel) above the bottom of the tank and draw water down through it. This feeds the nitrifying bacteria which grows in the substrate, which is the biological filter for the aquarium. The substrate also acts as a mechanical filter As the tank water is drawn down through the substrate, it traps the suspended particles, taking them out of the water.
UGF's work well when you want a reasonably good filter in a tank that you don't want a lot of water movement. We have found that UGF filters work very well for raising Clownfish or Seahorse fry. You can use an air stone in the uptake tube to create the water flow down through the filter, or you can use a small powerhead to create a stronger flow through the filter and water movement in the tank.
The UGF illustrated in this DIY project was created for raising newly born Seahorses and uses a small Hagen Aquaclear powerhead both to drive the UGF in the 10 gallon tank and to create a bit of water movement. The parts list is for a 55g tank UGF. You can adjust the amount of PVC and screen to fit the needs of your tank.
Ok, here is what you will need to construct your own UGF filter(s) as we did.