Based on much reading, and first-hand experience with a Niger, I believe it is just a matter of time. Once they get big enough, they will start eating inverts. I removed my 5-inch Niger a year ago and it is a whole different tank. Less stress on the other fish (they are scared s&%^*less to be cooped up with a predator). There was less shoaling from chromis and dartfish, but then again they were shoaling because they were terrified. All in all, a much happier tank without the trigger. Mind you, the trigger never ate any fish. But prey fish know perfectly well what a trigger looks like, so I am not sure it matters whether or not it is an aggressive one. My chromis and dart fish don't read Reef2Reef forums, after all...