I'm posting this for a friend so I can't verify all this data is correct but I'll try.
He has a 55 gallon planted tank (not sure of the plants) with 8 danios (different types) 6 emerald green cory's and 2 molly's (one silver and one black, both male) and is planning on adding 4 female molly's in the future. He started the tank about 3 months ago and is slowly adding in new fish. Tank has cycled, nitrites/ammonia 0 nitrates 5 ppm. 135w of 6500k lighting, emperor 400 bio-wheel filter. Tank is heated (don't know the wattage).
The black male molly is a recent add (about 4 days). It seemed alright on day 1, and since then he barely moves. He more or less swims in place with his fins moving very fast and breathing very heavily. He has been at the top and bottom so it's not like he's just gasping for air at the top. No other fish in the tank have displayed any of these signs. He doesn't eat. There are no signs of any of the other fish acting aggressively towards him. There is no visible inflamation of the gills and no visible signs of any other damage to the fish.
A) Is this a disease or is it possibly just stress? B) Should he be moved to a QT? C) What treatment should be used?