I am using a tub, it's approximately 31in×15in×13in ...Will a 75 watt be strong enough for those dimensions? Also I'm not sure if I should put it so close to the heat source itself because I want the whole tub to warm up. My thermometer says the temperature is at 72°F while the Thermostat heat detector states 69° Both are on the cooler side of the tub but are across from one another