Keep him in the warmest part of the house. If you have a fireplace or wood stove, bring him into that room. Otherwise, you can wrap the tank in blankets or towels to keep as much of the heat in as possible.
If you have a way to heat water, you might want to do very small water changes if it gets too cold for him.
Good luck and stay safe!
Animal testing is a terrible idea; they get all nervous and give the wrong answers.