Funny fish???

Hi. I'm very concerned about one of our GF. It struggles to swim, eat or even stay afloat. He occasionally moves around, including jumping straight up out of the water! He'll swim briefly, and then just slide across the bottom of the tank. I haven't seen his eyes move for a while, and he lays on his side like he's dead. The other fish simply shove past him. I think he even went belly up at one point! I've isolated him from the remaining fish, hoping that fresh water, less light and noise will help him out. I've tried to search this peculiar problem online, but nothing that addresses all the issues he has.

On a different note, our shubunkin (Fatboy) has gone quite red in the last 36 hrs. We moved the tank into the front room, and gave the whole thing a thorough clean out. (No chemical cleaners, just water.) They have no gravel, and only a plastic plant that they had before the tank clean (which was also thoroughly beaten and brushed). We bought them a brand new filter pump, which was a nightmare to set up. Fatboy (Shubunkin) was very happy in the clean tank with fresh water, before the new pump was attached...I'm wondering if this is what's caused his sudden discolouring??

Any help or advice would be appreciated, as we are novice fish keepers. Feel into the whole thing by helping out a family friend!