please help my blackmoor is very sick

hello all

i need help urgent, i have 3 orandas and a blackmoore in my tank, 2 days ago i did a water change and they all seemed fine but today my poor blackmoore is just lying on its side and floating around, when i feed them this afternoon he did move and went for some food, but after finishing it's just floating on it's side again.
the water seem to have gone cloudy since the water change and the orandas are just fine but i really don't know what to do to help blacky

thanks for your help :sad

it is a new set up, but i did the cycling for 2 weeks as the man in the shop told me to and i introduce the fishes in a period of one week, (first tank, call me anything )they have been in there for three weeks now an the water has always been a bit cloudy which the man in the shop again said it was normal and not to worry about it, but i do now as i do not want my fish to die on me, mind on the last water change, it went more cloudy than what it was before.
is there any thing i can do for the blackmoor.

thanks for replying