green spotted puffer has yellow spot on forehead

[xhaust50]

I just noticed today that my GSP has a yellow spot on his forehead, with a whitish outline. I took pics, but my camera stinks. Any idea what it may be? He also seemed to stay a little fuller than normal the last 2-3 days. Other than that, he seems normal. I have had him for about 18 months, slowly converted him from brackish water.

Background:
55G, 55lbs Live Rock, Sand Substrate, AC110 Converted Refugium, Skimmer, 32X Turnover, Inhabitants Include the GSP, a Flame Angel (being transfered) and a Dwarf Zebra Lionfish, all about 3.5". No new additions to the tank, and no aggression between anything. The puffer does not go near the lionfish's fins for those who are about to ask, he is really quite docile. Everything else seems healthy. The tank is fed once a day, a frozen mixture of squid, shrimp, clams, mussels, oysters, nori, selcon, and garlic.

Tank Parameters:
Temp 77
SG 1.025
pH 8.2
Ammonia 0
Nitrites 0
Nitrates 5
Ca 480
dkH 12

The Ca and dkH are on the slightly high scale but I just dosed today beforehand, and I don't think they are relevant to the puffers condition.

The spot doesn't appear to be raised, atleast not noticeably. His eyes/coloring look great.

Crappy pic:

 

[xhaust50]

Is it possibly a sting from the lion, and what are the consequences if so?

 

[Chad Vossen]

i would go with hole in head disease. though this is what poped into my head while reading your discription.

 

[xhaust50]

No I've seen hole in the head disease, that's not it. Sorry the pic is so crappy. On the plus side, it looks smaller today. I'm thinking he might have been stung.