Ick looks like tine salt spots, dosen't like that to me. Is the mark raised at all and have any texture to it? What other fish do you have in the tank? Tanks size? Post all the params of your tank. What do you feed him?
No raised marks or any texture. It looks pretty smooth. It's a 120 gallon tank with 2 other tangs. I feed him frozen shrimp, krill, and brine. Parameters of the tank is good. I've had the 2 tangs for over a year with no issues. The puffer has been in the tank for a couple months.
Oh I just noticed something....every time I stand in front of the tank he likes to press his mouth against the glass and his mouth turns white that is very similar to the white mark. Perhaps he pressed his face against the glass too hard and caused the injury? If so what can I do to help him?
Nothing you can do about what a fish does in a fish tank. We need to assess how or if this develops further. He may have been leaning against some rocks at night while asleep that caused an erosion pattern.
What are your water specs and what and how often are you feeding your fish?
Your temp is low, I would raise your temp. to 78, but slowly. And your salinity is a little low the big blue on the average is 1.025 or 35g/l. Not that any of these changes will help the white spot. But in the long run may.
I have a spiny puffer that has developed "large" white spots on his/her back. The number of spots are growing every couple days. I treated it with an antibiotic - but that didn't work - as the spots continue to increase. Looking at the photo (attached) it appears the spots may be some kind of...