Looks OK to me. Convict tang is nice and plump, and they are often thin in captivity, so I'd say you are feeding the tank just fine. My red sea regals always ended up losing a bit of the black edge in favor of the blue, so maybe it's a 'normal' reaction. A stunning fish regardless and as long as it is eating and maintaining a healthy body weight I'd not worry. BTW, I'd say you've done a really good job with this fish. It looked very thin in that year-old picture, and now looks to have a very health body weight.
