Tinkerbell, I too have the same issue with mine. I didn't think that it has been there since I have had him (about 3-4 months) but the rest of his color is fine and the same as it has always been! I think keeping a close eye on him is the best thing for now. I've grown really attached to mine, much more so than my other fish, so I think I might just be paranoid (plus he's the first betta I've ever had)