As a general rule people tend to be a little dogmatic on these kind of things. Fluidized beds are no more nitrate factories than is live rock or deep sand beds. Almost every public aquarium runs these in addition to protein skimmers. The size of these filters are built to the relative volume of water housed in those facilities. They are however several orders of magnitude more efficient than than either live rock or dsb, or for that matter both together. The use of the term sand in Fluidized Sand beds is misleading as many different materials are used. Many other materials will facilitate a more thorough backwash of the system. You will quickly find these on the intenet. To answer your question on what happened to them in the home aquarium reef: They have morphed into chemical media reactors which depending on the media accomplish the same goal as the original fluidized sand bed in addition to simultaneously removing the nitrate at the end of the process. In my own case, I actually don't worry much about whether the bacteria are grown in the media because I actually fluidized Purigen, but I digress. Hope this helps.