I've never seen one go after fish, but when they got big they do go after soft corals.
I bought 5, all of them small. I put them in my 34g and apparently, that was too many. One grew really big and killed a couple others and proceeded to munch on my Pom Pom Xenia. It took me forever to catch him. I had named him Godzilla.
Stay tuned go the 40b!!!