help with a purple firefish

[amazo]

I just got a purple firefish and when I got him home I noticed a weird bump on one side of his head,it looks like a blister near the back of his head it stops at his gills.You can only see it from certain areas but is a pretty big bulge,if you look at him from the side it looks kinda red but deep down becuse the bulge its self is "clear" like a blister.He seems fine and is eat good I just want to know if there is something I can do for him or just leave it be.I tried to get a picture but becuse you can only see when he is at the right angle they didn't turn out.