I'm wise enough not to add a trigger into my reef, although in very large tanks, it has worked to some degree. I just wish people would be honest about their mistake, when that trigger matures. I witnessed a friend being totallly dishonest at an lfs, after seeing with my own eyes what a clown trigger did to his reef. It was a trend around here about 10 years ago that ended with alot of extremely mad people, one lfs that pushed it lost alot of customers after many devastating results. These fish have teeth, and you can find articles about their intelligence and need for enrichment. That's why many of them become so destructive after a while.
Niger's have a tendency to wedge themselves under the rocks, fanning their tail and causing sand storms; disrupting, over turning, and yes even picking up and carrying corals across the tank. I've seen nigers with rocks almost as big as they were in their mouths. Be careful following reports of juvi's, sub adults, and new additions. Give it time, when that niger reaches 6" plus with visable teeth, you are talking about a completely different animal.