Since you cannot fit a heater, there is another solution. At Walmart, buy some black fabric. Something thick that will hold heat pretty good. During the daytime, shine a bright lamp over the tank to heat up the water. At night, put the black cloth over the tank and shine the lamp on. The cloth will hold heat since its black and heat up the water a bit. Also since its black, it should keep some of the light out so it can sleep. You have to experiment, because I'm not sure how much it will heat up the water.