Mostly an experiment... I have a well insulated cooler in my backyard with about 8 gallons in it. Next time I change the water im going to lower it to about 6"-8", about 4-5 gallons. Where it is it will get a good amount of sun during the day, and I haven't yet established the heat schedule and how the day will cycle, I'm just hoping it works. Right now its only 70F, but by the time i had it ready to go it didn't have much sun to absorb. Hopefully tomorrow will run a 75-82 average. I have two "gold" gouramis, hoping they'll breed for feeders. If they die I'm going to restart with a heater installed. Any past experience will help.