All they are is a basic LOW FLOW fluidized media chamber. The most basic, effective, least costly one is the TLF Phosban Reactor 150. Seems to be an industry standard.
I just made one like in chiefs thread. Actually mine is a lot uglier, and I made the holes too small so the water squirts out like a fountain Oh, well, I put it under the water so no problem. I'll test the tank tomorrow to see if there have been any results.