two things may have caused it - (1) like above he tried to jump out and lost a few scales in the process, this is not really cause for concern. just check your cover and make sure it is going to keep him in there. he will eventually learn to quit doing it cause it hurts, unless something is frightining him into doing it. (2) he is getting into fights with tankmates (if any). if he doesn't have any tanksmates, then it must be #1
best specualtion i can provided based on information available.
his scales will grow back - takes 1-3 months depending on size