I wouldn't do sun... That is too unpredictable in my opinion. When you say tank do you mean you have him in a terrarium or aquarium set up? Even if it's a hospital set up? I won't tell you, you need a screen cage especially since your buddy is so sick that a glass cage might be more helpful at the moment, except with the respitory infection you have to be extremely careful not to over mist or allow it to be too wet. However, usually I add a small ceramic heat emitter, at a low wattage, so I can keep it on at night, if a vet says to keep him warm. Usually I try to get around a 40 watt or 60 watt, and depending on the cage set up decide what to do from there. I usually still prefer to get a ten degree drop in temp at night, but often houses can drop 15-20 degrees at night, with a healthy cham that is usually not a problem. But though you still want the benefits of a ten degree drop I wouldn't want to do much lower if I could help it until he's healthier.
