I would say that a thermometer in the water could help you ascertain how fast the water is heating up. If the tupperware came with a lid, you could use that to cover half of the container. If the water still heats up too fast, you can cool the water by putting frozen bottles of water in the container. I use 16 ounce water bottles with the labels removed. The thermometer will tell youif you need to add more, and the turtles generally leave them alone too!