Most of the normal fludized reactors tend to have issues with getting enough flow through the reactor to tumble the heavy pellets correctly. The best one I've seen is the re-circulating one from Reef Dynamics, but you have to sell your first born child for one
Recently I saw a post on ReefBuilders about the Hydra Aquatics bio pellet reactor. You can find them at Marine Depot. It works FAR better then the fluidized reactor I was using before