I realize this is an old thread but just experienced the same problem with the same fish (acanthurus sohal) and same coral (cataphyllia). My wife witnessed him take a decent size bite of it, including some tentacles. It was very healthy and opening nicely, now of course is ****ed off. I've temporarily covered the coral with a glass bowl supported by risers, so there is adequate flow, but the fish cannot reach it.
I've also noticed polyps from one of my euphyllia corals (branched frogspawn) floating in the tank, and twice have witnessed the same Sohal tang nip at it so he is the culprit. He did some damage to it so it is now relocated into the sump where it will hopefully recover. I have also seen him bite off a mouthful of green star polyps and nip at a neon sinularia. There is no question he went after the corals in these cases, rather than picking at adjacent algae as there was none and he had coral flesh in his mouth. I also saw him pick at a zooanthid rock, although in this instance the rock (a new addition) had a bit of algae on it so suspect he was after that. I've taken a photo of the zooanthid rock and will watch carefully for polyps disappearing.
Anyway I first got this sohal tang as a juvenile back in 2007, so he is 8-9 years old now. He is 8+ inches if you count his tail/streamers. He is a massive eater, a true pig, but fed
very adequately including nori hung several times daily in a clip and always available in an AUQA grazer, plus whatever he can grab from LRS frozen food fed to the other fishes... plus occasional chunks of grocery seafood that he steals from the mento triggerfish, who is no sloth but yet considerably slower. The sohal is incredibly fast and very efficient at hogging food.
I'm hoping it is a passing fancy as I stock the tank with new corals, but if he continues I'm not sure what to do. I really don't want to give him up as he is gorgeous, I've owned him since he was a juvenile, and as these guys go he is reasonably mellow - he pesters the naso tang occasionally but generally leaves all the others alone and has never killed a new addition. But if he is going to continue to destroy corals then he's gotta go
While it's not easy to catch a fish in a big reef tank, strangely if I set the LEDs to red only he goes berzerk, turns almost black, and cowers in a top corner of the tank! If I am prepared with a net I think I can get him; the other fish do not seem to care whatsoever about the red light, but for some reason it drives him absolutely crazy.
Interestingly, despite unquestionably being the tank bully, he has gotten to taste a bit of his own medicine as the smaller naso is much more adept at using her tail as a weapon, and the laid back triggerfish knows how to use his teeth when he is getting pestered.
