Does it fight with the other fish in the tank? Looks to me to be a war wound. I had a male jag that jumped out of the tank and almost cut its bottom lip OFF. Lots of water changes maybe some metaflix(sp?) - I had some good results with this on some fin damage. But most important if the fish is getting picked on you wil need to keep a close eye, once a fish has started to get picked on and gets injured it usually will get killed by the agressor. If you have a Q Tank I would let your rainbow heal and then reintroduce into the main tank. Also some dithers/Target fish will take the aggression away from your "main" fish. Tiger Barbs, Giant Danios, Silver dollars.