Trigger help please

I noticed that just behind the yellow fins on my trigger it seems to be kinda sunken in and looks like some scales are missing, but it is the same on both sides in the same spot so i don't think it is from hitting something.the rest of the fish is fine. he is actually kinda fat. he eats a lot and is swimming normally. could this be a disease or parasite? what should i do?

 

Can tell form the pictures but from what I remember that is normal. its sunken so they can lay their fins flat against their bodies.

 

It's hard to get a good picture, he swims a lot. The sink seems to be normal from other pictures i have seen but what about the discoloration?
Thanks for the reassurance.

 

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