Does anyone still use fluidized bed reactors?

Oh yea sure. The Berlin system is the use of brilliant lighting, live rock, and a protein skimmer. This system was discovered by a German named Eng who wrote some articles claiming removing his trickle filter and adding all what he called "wild rock" aka live rock reduced his nitrates to zero. So Germans published that adding live rock, light, and a protein skimmer lowered nitrates from little with trickle filters to none without.

So it became known as the Berlin system.