As a fluidized bed filter performs purely biological filtration, they must be used in conjunction with a mechanical pre-filter. Fluidized bed filters are columns of sand held in suspension by a flow of water from the tank. Bacteria flourish on the surface of the suspended sand and process the nitrogenous wastes. As the sand particles tumble and collide in the flow, the old bacteria are dislodged and replaced by new growth. This filter has the great advantage of rarely needing any form of maintenance and therefore the concern of removing the vital bacteria population associated with conventional filters is resolved.

Fluidized bed filters are designed to stand outside the tank so space must be available if you wish to install one. Some manufacturers design these filters as part of a modular system, each module in the system designed to perform a different filtration task such as mechanical filtration, chemical modules and even heating modules. These systems can be a good alternative to building your own sump filter.