Well the extra air supply looks to be working. All of the fish seem happier and they're all feeding well. One is still heading to the filter return regularly but he's also socialising and eating food. I'll be keeping the extra air going in for now, especially with temps heading back up next week. I now need to get the balance of food right. Digestion uses lots of oxygen so I don't want to pile the grub in for growth until the water temps are down a little.