Bloated Chromis Help

I have a relatively new (1 week in QT) blue green chromis that has been acting a little sluggish the past couple of days and today he's looking bloated but only on one side. He's been laying up against the rocks (but not scratching, etc.) and hanging out at the bottom of the tank not swimming much. His mate we got at the same time is perky and swimming around like crazy.

Water tests are normal (Nitrate less than 5, Nitrite 0, Ammonia 0, pH 8.0 and temp 78). 36 gallon quarantine tank that has been established for 6 weeks.

 

Could have internal parasites, what do its feces look like? Are they stringy and white? Has he eaten?

 

He hasn't been eating much. He did have kind of stringy poop hanging the first day after we got him home but haven't noticed anything abnormal since. I think he's almost gone as his scales are now protruding and he's just laying around. Wondering if I should preventively treat the other one or just wait and see. From what I can tell there are no symptoms on the other one.