My Betta Merlin did this too when I first put him in his tank; I put some white paper on the outside, but nothing seemed to work. I asked for help and here is the best reply I got:
Originally Posted by Bluewind
Gus did the EXACT same thing when I moved him to his new tank. It got better with time and by day 3 or so he was like Meah, whatever. So don't worry. Just make sure he eats enough to compensate for all the energy he is burning being macho.
I made sure he was eating enough and after a few days he calmed down. yesterday I was able to take the paper off, and when I did he had no reaction.