I don't believe it's normal--not for my convicts anyway. Something is irritating him. Could be the beginning stages of Ich. If it's Ich the small nodules will irritate his skin and he will scratch against surfaces to get some relief. Keep an eye out for the white spots. I had good results with a bout a few months ago by increasing water temp, increasing salt , and using Rid Ich--following the instructions to the letter. Im sure other issues could cause the irritation but Ich is the only one I have had experience with. Good luck.