basically thhe same thing as a undergravel filter, instead of sucking trash down through the gravel, it pushes water under the gravel and back up into the tank, hook a powerhead backwards to a ugf and you have it.
not exactly but does about the same thing just without the sand and can be done in tank, here one you migt be interested in, i've looked at a lot of the diy's on these. http://www.aquariumlife.net/projects/diy-filter/10.asp
let me know if that helps any, i have more links saved somewhere
all they really do is push the water up through sand which agitates the sand and offers a large surface area for bacteria to grow on. basically, here's a link explaining how they work http://www.bioconlabs.com/abtqs.html
Heres what I use and you can often pick them up new on ebay from 20.00 - 30.00 i have 1 on each tank I own:
Red Sea Merlin Fluidized Bed Sand Filter up to 200 gallons. Water is supplied to the Merlin by a 150 gal. per hour, 5 ft. head pump (not supplied with the unit) and placed in the aquarium or sump. The water flows into the fluidization chamber through the central inlet tube which includes a non-return valve to prevent back-siphoning of the media. The water flows up through the fluidization chamber, fluidizing the media and is returned to the aquarium or sump via the outlet in the lid. Flow regulation: Rotation of the Flow Regulator (on the tope of the unit) sets the opening between the inlet tube and the fluidization chamber which determines the flow rate of the water through the unit. The fluidization level, therefore, can be adjusted according to the pump used and individual install