I have 2 x 650 liter water storage containers in my green house. I am going to spray paint them with black paint. I am also going to coil copper wire inside the tanks of water. Copper is a good conductor of heat and it will not harm the water. So the water in my 2 water tanks should get hotter. And when the temperature drops in the night, that stored heat should help heat the green house. This will not help much in the winter, but I’ll get better use of the green house this spring, and in the fall. At least this is the hope.