Hmmm, well I'm no expert on filtration but for a 55 with mbuna, even at 18 and adding catfish I can't imagine needing a fluidized reactor. There's lots of threads here about people trying to figure out maximizing surface area and cramming as much biomedian as they can into a filter, but from my limited experience, and comments from other very experienced aquarists such as
duanes
, i believe once a tank is fully cycled it's a fairly small amount of biomedia actually needed, the bacteria only expand to the size of bioload provided, and they won't expand beyond that despite how much media is there. If you're still having ammonia or nitrite readings after maybe 6 months of operation then perhaps you need more biomedia, but less time could mean your cycle just isn't complete, or if it's nitrate you're worried about, bio won't help. Water changes and plants are the only solution. Or if water clarity is the issue then you need more mechanical filtration.
Anyhow still a cool experiment and contraption to get working, but You might need to clarify the specific filtration issue to see if you're solving anything.