Mini-catalytic heater for keeping tanks warm off-grid??

DW suggests a 12 volt LED bulb, like the ones in the inside fixtures. (The bulbs emit some heat and use almost no wattage.) This would be inside an enclosed space, right next to the outlet tube coming out of the fresh water tank, and we would turn it on and off manually. I would monitor the tank temp to see how much this affected the water. Might not be enough warmth. But all I want is minimal effectiveness, not hot water!