Maybe add some fake plants in the 2.5 to crowd it up more so he can hide in them or explore them instead of biting? You can block off his tank from other fish so they can't see him using paper or those aquarium backings, scrapbook paper for pretties, or just a notebook [whatever you have lying around]. Try that for a day and see if there's an improvement. If not, then something else is stressing him, whether it be water changes, water temp, something IN the water, something in the tank, etc.