A 4x5 might be a little small for a 41qt tub, but I think it would probably produce enough heat. Alternatively, you could pick up a heat pad from a pet store, or order some flexwatt heat tape online. With any heat source you use you must use it with a thermostat to keep the temperatures controlled and constant.
Not to mention you also need to have an accurate thermometer to be sure the thermostat is doing what it should be.
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