Fluidized bed filters were developed in aquaculture facilities to cope with high densities of fish. Although their design varies, the concept is quite simple. An acrylic or PVC tube is filled with fine sand. It is usually hung on the back of an aquarium or sump or is freestanding within the sump. The tube is either sealed at the bottom or both ends to form a pressurized chamber. Water is pumped to the bottom of the tube, forced upwards through the sand and exits an outlet at the top of the tube. The sand grains allow for an enormous amount of surface area for nitrifying bacteria to colonize. As the sand grains and water swirl in the tube, bacteria are fed and some scrubbed loose from the surface of the sand. This ensures that new nitrifying bacteria are continuously growing on the surface of the sand. Young, rapidly growing colonies of nitrifying bacteria are much more efficient at transforming waste products to nitrates. The constant grinding of the sand ensures the bacteria stay in a state of exponential growth. These filters have proven valuable to public aquariums or central water systems with high waste output from high densities of fish. Dangerously high ammonia levels will be reduced to unreadable levels in a few hours when a seeded fluidized bed filter is added. Fluidized bed filters have become very popular with home aquarists of late although those for the aquarium hobby differ dramatically from the huge units used in commercial aquaculture.
Activated carbon is created by exposing carbon grains to high temperatures and pressures, resulting in a very porous material that functions much like a sponge. Activated carbon works in two ways to remove organic molecules from the water column. It traps organic compounds in its pores and chemically bonds polar organic molecules to the surface of the carbon grain. Activated carbon can be used in a variety of ways depending upon the type of set-up being used and the room available. The best results occur when water is forced through the carbon so that no water has a chance to bypass it. Carbon can be placed in mesh bags and positioned under the water inlet to the sump or in an inline chamber. Inline chambers can be created from PVC pipe fitted with reducers and tubing and supplied with a piece of foam at both the inlet and outlet to keep the carbon in place. Canister filters can also be filled with activated carbon and fitted on the sump. Bags of carbon simply placed in the sump will take up organic molecules, although at a much slower rate.