The best bet would to be to get a heater, it might use a lot of electricity but it gets the job done, to maintain a 40 degree tank would be impossible the air around the tank would make it still warm even if you did have a cooler for a tank witch i don't think they make.
Yoy can make it very easily. In a natural way you can maintain your water heat.
YOUR IDEA IS APPRECIABLE ONE !!!
I'm trying to think of creative ways to heat and maintain water temperature in a 50 or 100 gallon fish tank without using a ton if any electricity. I'm likely going to set up more than one tank. One will house fish that do best in 75 degree water and the other tank for colder water fish no colder than 40 degrees. I'd likely keep the tanks in different spots to maximize each tank requirement like a garage, greenhouse, and/or outdoor non-insulated shed.
Is there a realistic way of doing this?
