Hi I have a 180 litre tropical aquarium with one 200 watt heater, I want to put another heater in the tank set at a slightly lower temperature just incase 1 heater fails or breaks! Would it be ok to add another 200 watt heater or would I need to get rid of my 200 watt heater and buy two 100 watt ones, please help because I don't want to harm my fish or cook them with too much heat! Thanks