Fish losing weight/mass will show it first in their belly, and then in the nape of their neck like this one. It generally means that calories going in are not sufficient to maintain proper body mass - either not enough food, not being fed frequently enough, the wrong type of food, or competition in the gut with internal parasites (like a tapeworm).
There is one other cause for a RAPID thinning of the nape like this - due to dehydration if the fish is suddenly exposed to a higher salinity. I see this a lot when people buy fish from the LFS and the fish are held at low salinity, and then, they take the fish home and put it into a tank with full strength seawater...the fish cannot maintain osmotic balance, and the fluids rush out of its body, causing almost immediate thinning of the tissues. Your timeline doesn't really match up to this, but I wanted to throw it out there just in case.....
Jay
