I have just the one in there.
As for nibbling, well some days I am convinced it is eating my sps but have yet to see a real sign that it is doing nothing more than catching bugs on the base of them.
Often it will hover and it seems like it is looking for something, then it will take a bite and swim away. It doesnt appear to graze upon any coral.
It did eat my open brain, but that was a long time ago. It ahsnt touched or shown interest in any LPS since.