Just stumbled across this thread. I'm not convinced they need a dirty tank to grow. I've got a fairly clean tank (0.0 phosphates, 0.2-0.5 nitrates) and do weekly 10% water changes and they grow like weeds. Bought a colony 3+ years ago because I thought it would be cool to have little patches of them throughout the tank. Like others have said, watch what you wish for. One day, I noticed a bunch of white things on the polyps and went to blow them off. Didn't realized it had spawned! Ended up with the stuff in every nook and cranny in my tank, even growing on the silicone in the corners. It *is* nice, and seems to play well with other corals in that it's always the loser!
I've noticed lately that it does seem to be going away in some spots where it used to be really dense. Not sure what is going on, as nothing has changed in my tank. Maybe whatever trace element it feeds off of, it's finally depleting and I'm not replenishing it quick enough? Don't know. I'm dosing a fair amount of c-balance everyday. I run 2x 96W PC lights, and it grows everywhere - top of the tank on the back glass, and in the sandbed.
Not sure what it likes, but thought I'd relay my experience as another data point.