What is wrong with my puffer fish?

I have a green spotted puffer, and I've had him for about 6 weeks and he's been fine up till yesterday. He's usually very active, however, since yesterday he's been very still, resting on the decorations and the bottom or just floating in one spot and he's belly is a very dark shade of gray. He is not very eager to eat, he used to wish he can swim through the gl, today I had to tap the forceps on the gl with food in it for nearly a minute before he decided to swim up and eat. And he usually ate each of the blood worms in 1 gulp, now he's eating and spitting out. He is breathing very fast and he twitches every minute or so. I do suspect parasite but I am unsure. His feces is long oval, reddish brown when he first poops. I keep him in low brackish conditions, currently at 1.002, slowly raising. The ammonia is almost zero. Can someone tell me what's wrong with him and what I can do?