That dent could have been caused by nothing more than another fish accidentally running it to it. Unlike a car that dent should repair itself in no time. Fish seem to have extra fast healing powers. I had a large Blue Scorpion jump out of the tank the other day. While she was flopping around on the floor, and before the family cat could get to her, I sprang into action, beating the cat to the misguided fish. In the fall she injured her tail fin. In two days time the fin was fine.