How would you tell if a stars and stripes puffer was sick?

[jassz]

Kiwi, our stars and stripes puffer, has periods of time where his colour turns pale, mottled and rather ghastly looking. It looks really unhealthy, but the prev owner said that he does look quite terrible at times so maybe it's normal for this fish? Is he trying to camoflage do you think, or is there something wrong? My dog is OBSESSED with this fish and has spent hours and hours staring at him (I think he thinks Kiwi looks like his tennis ball, with wings! ). I thought maybe the dog was stressing him out, but oddly enough, Kiwi actually comes OUT of hiding spots to sit in front of Maverick. They gaze longingly at each other by the hour, occassionally Maverick with lick the glass. Theirs is a strange and wonderful relationship. Kiwi is eating, but not at every feeding, and he does have periods of time when his colour seems to return to his normal tennis ball green. BTW, that is a bit of half chewed food sitting on him, not skin.
Oh, and his eyes change colour too. Sometimes they are turquoise, sometimes yellow and sometimes black!