NEED HELP!!! GSP PROBLEM!!!

just got a green spotted puffer 2 days ago, has been doing fine until today. He goes in and out of normal behavior and his colors are inverted! I've never had a puffer of mine do this before, and i've many over the years, any help will be much appreciated.

and yes he does eat, and he seems fine when doing so

anyone?

I have never had GSP's so I cant help you. Have you checked the water chemistry? Are there any other fish in the tank? Those would be the first thing I would look at since normally color changes (to me) would indicate some sort of stresser.

water is at 1.010 on the hydrometer and there is only one other gsp, got them together and they never leave each other's side. water has only had the two in it for 3 days now, so waste levels are low

Color reversion usually if I'm not mistaken in brackish fish is a growth sign Sort of like the teenage years in humans, Don't quote me folks I haven't dealt with a brackish tank in over 25 years.

what were the water conditions where you got him at???is pH the same did they add salt...3days you can spike ammonia unlikely but poassible. How long is the tank light on for. Is the water temp the same?? All of these factors can stress the fish out.

What do you mean by goes in and out of normal behavior..is he doing head/tail stands (bad sign)? Does he show rapid breathing? Is the other puffer nipping him?

I have heard that young puffers can have this color swap happen before as well, and as far as i can tell that must be it.

As stated before, the water levels are tested and right on, and they do not pick on each other whatsoever, the "in and out of normal behavior" i was talking about was the color changes and normal activity/ inactivity swimming around the tank. His buddy shares none of his symptoms and appears to be normal and healthy. The light is on 10 off for 14. water is the same in both the brackish as well as my freshwater as the shop which I buy all of my fish from. water temp is one degree warmer.

update:

so now he is swimming around, but his back fin is almost always curled to the right and his back fin seems to be turning a dark color. could he have internal parasites?

Oh not good! could be parasites. Sound more like disease than water conditions.

i hate buying sick fish.....

I just googled green spotted puffer diseases and it sounds like stress related. Recommended treatment in most sites are tertacycline or maricyn-2 to prevent infections or treat infections already started from stress.

ya i've seen those but i feel like it may be internal since he seems normal otherwise and his buddy is totally fine

His buddy may not be stressed. What are your parameter level (Ammonia, nitrite and nitrates)?

all are perfect with a ph of 6

From my understand they like higher PH of 7.5 or above.

i'll bump it then and see if that helps

update:
observing him from a distance he is fine, great color and very active. it seem that only when i'm up close to the tank he shows these signs of stress or sickness. could it be me just being close to the tank that is stressing him? i don't do anything that would stress him other than looking at him. this is without a doubt the weirdest puffer i have ever had.

update: his tail fin is now turning black and he remains a grey-ish color. he doesn't have a caved belly or black-belly and he eats just fine. the rest of the time he just sits on the bottom of the tank. His friend is still doing just fine. Water is still spot on. what else besides stress/parasites could this mean?