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I've kept tilapia alive over the past two winters here in the center of Tucson. With a closed and thermostatically controlled greenhouse, you can set the temperature inside where you want during the day. Without thermostatically controlled ventilation, the temperature in my greenhouse would get so high it would kill the plants inside. I typically set it at 85 degrees, where it stays for most of the daylight hours. Inside the greenhouse I have pvc grow tubes that heat the water that flows through by two degrees, which raises the water temperature during the day. I also have a large (700 gallon) tank in the ground and one (550) above ground, along with the grow bed they make for a huge mass to hold heat, so less heat loss overnight. Typically, the water temperature ranges from 55-65 on the coldest nights/days of the winter.