hah, fluidized bed filters are awesome. I wouldn't build a clients tank without putting one on it.
Fluidized bed filters use sand. Highly oxygenated water is pushed down a tube and up through a sand bed, keeping it fluidized. Since sand has so much surface volume, trillions of aerobic bacteria build up on the sand and break down nitrates, phosphates, and other organic compounds that are in the water.
Generally 1 lb of sand can handle 100 gallons of an average stocked aquarium. Although, I run most tanks with 9lbs of sand, if they are greater then 200 gallons.
Larger fluidized bed filters for 1000g plus are freakin sweet. They are round and have the same basic principles as the smaller ones that you can buy online. Generally you can run around 50lbs of sand in them, (which is for about 50,000 gallon aquarium)
I will be running a large FSBF on both my freshwater and saltwater setups for my storefront.