And from http://www.aquariumpros.com/articles/biofiltertypes.shtml
Fluidized-Bed Filters: Fluidized-bed filters are a very unique biological filter. If properly designed and built tall enough, they have the ability to cultivate not only aerobic nitrifying bacteria, but also facultative anaerobic denitrifying bacteria. This means they may have the capability to remove not only ammonia and nitrite, but also nitrates. They usually consist of some type of column chamber which houses several cups or more of coarse sand or similar media. Water enters at the bottom of the filter and exits at the top. There is usually a control valve for regulating water flow. A check valve is usually placed on the filter intake to prevent the sand from packing-down when the filter is turned off.
Because water flows upward through the filter, the sand in the filter becomes suspended or "fluidized" in the water column, forming a fluidized bed of sand. If the flow of water is controlled properly, the sand does not flow out of the filter, but remains suspended. This happens because the flow of water is just fast enough to keep the sand in suspension. The weight of the sand prevents it from escaping the filter. Because the sand is suspended in water, fluidized-bed filters are self cleaning, and require little or no maintenance.
The water at the bottom of the filter is fresh and high in dissolved oxygen, so aerobic bacteria cultivate in the bottom half of the sand bed, and remove ammonia and nitrite, using up oxygen in the process. In taller fluidized-bed filters, enough aerobic bacteria cultivate in the bottom half so that as water flows past them, they remove most of the oxygen from the water, so facultative anaerobic bacteria cultivate in the top half of the sand bed where they remove nitrates. Not all fluidized-bed filters are tall enough to promote anaerobic denitrifying bacteria, but most are very efficient at cultivating beneficial aerobic bacteria.
Some fluidized-bed filter designs are stand-alone units that are too tall to be placed under an aquarium in a cabinet stand. These are actually the best designs, but are not practical for most aquariums. Other fluidized-bed filters are designed to hang on the back of an aquarium or sit in a reservoir, and may be driven by a small pump or the return line of a canister filter. These types of fluidized-bed filters are an excellent way to provide biological filtration on an aquarium equipped only with a canister filter.
This is exactly what I'm talking about:
If you look at that, it's just 4" tube with 1" tube inside of it and an outlet coming off the 4" tube and an inlet on the 1" tube. It's an external reverse flow under gravel filter!!!! And I'm just guessing, but if you have a strong enough pump and the specific inclination, you could run this setup in series... ie multiple 4" pvc tubes with 1" tube linking them together, and different filtration media in each....
Read the link info:
A DIY fluidized sand bed filter for an aquarium. A FSB is a biological filter with a surface to volume ration of 1000:1. The bacteria convert ammonia waste into nitrite and another strain converters nitrites into nitrates, which is exactly what plants thrive on. This unit can process ammonia for 18 pounds of fish. For more info see http://backyardaquaponics.com/forum/viewtopic.php?p=113250#113250
This was made for under $17 from LDPE using a plastic welder. A commercial version cost hundreds of dollars. This biological filter has a surface area of 4,430 square inches/ 9.309 square meters, enough to process 1.86 to 8.3 grams of ammonia per day in theory or based on the SRAC pdf, 2 to 10 grams of ammonia. I have room to add more sand, but that might make it unable to restart after being turned off. Southern Regional Aquaculture Center says 10g/day of ammonia is produced per 100 pounds of fish. So .1 grams of ammonia per pound of fish that would mean this filter can handle 18 pounds of fish.