Ask any hobbyist and you will hear the same answer. Fish have distinctive personalities, just as humans do. Some get along swimmingly with tank mates and others show hostility and aggression towards them.
Some are happy to share their space and others try to claim the whole space for themselves.
Into a calm, settled and friendly salt water reef tank, add a beautiful Raccoon Angelfish. Not only does that angel never stop swimming, but he does it an agitated manner. I’m sure it causes stress and rising blood pressure levels among the original members of the tank.
He looks pretty but his personality soon detracts from his looks. Although his looks may be pleasing, his destructive eating habits start exhibiting themselves. He immediately starts nibbling the coral. Not the algae on the coral, the way the other fish do, but the actual coral. Given the opportunity he will decimate the coral structures within a matter of weeks. The tank will resemble the sad dying coral structures of the Great Barrier Reef of Australia.
In the name of peace we make the decision to send him back to the store he came from. The Angelfish is unceremoniously scooped out of the tank and placed on a holding tank, where the owner of the fish store will pick him up and sell him another hobbyist with fewer underwater real estate issues.
We wish him well in his new home, but are grateful that harmony is restored to the peaceful, established tank that was there before the invader came along, and will hopefully remain that way for a long time.