Needeles said:You should be able to do this. Just make sure that you have something around the heater to allow the water to be moved throughout the tank. This way the heat will not just sit in one area of the tank.
Tommy Gun said:If you think about it, the heater cannot get really that hot since it is constantly transferring its heat to water. It would take a lot of heat to melt plastic, especially if it is in water.