I need to get a new home sorted for a single fish that's been bullied by his tankmate. At the moment he's recovering in a in an emergency 6x3 inch tub which seems OK temporarily, he's never been very active, particularly not at the moment though. I'm worried about the water quality - I'm changing it regularly with water from the main aquarium but he's going to need something with a filter long-term. I have an old spare tank but it's huge for just one little fish, and I don't really have anywhere suitable to place it at the moment.

Also, how sensitive are they to temperature? The room he's in is warm and he's quite close to the heater so I don't know whether he'll need a heated tank.