Personally I wouldn’t remove any fish at this point since they are all infected - you want to keep stress at a minimum. Unfortunately, I think the one that is hanging around at the bottom on his side might be too far gone – that’s what I observed in my tank and he was dead in less than 24 hours.
Just keep up with the treatment and invest in a heater that can reach the 80's, 75 isn’t going to cut it.