I would be very careful about aquarium heaters. Try to ensure that the heater itself is not touching the plastic- if it gets hot enough it could melt the plastic. Sounds like people have not had problems with lower wattage heaters. You won't need much, unless you have very cold winters, to keep the water thawed. Also, make sure you are refilling your bucket frequently with an aquarium heater; if the heater is not completely submerged, it could shatter.