Starchick, for a good amount of time, your apt is even colder than my basement, where I have two painteds in a stock tank. The ambient air temp varies somewhat, depending on the weather outside, and can range from just about 60F to 64F (during the recent thaw we had). I try to keep the ambient air temp up in the tank---it is possible for them to get ill from the contrast water and air temp.
I keep about half of the tank covered with a piece of Lexan during the day, the other half, where their basking area is, has a 160-watt MVB over it. The water temp is set at 72F. At night, after the light goes off, I cover just about the whole tank (and piece of Lexan) with a big piece of cardboard to trap in the warmth. One of the painteds likes to sleep on the basking area and can usually be found there in the morning. So far this has been working well. If I place my hand under the cardboard during the night, the warmth is really noticeable.



