Stick-On Pad Heater for Tanks EH-3659 250 Watt 12V DC Stick on pad heater 5 x 5 inches, heats 10-30 gallons. Pictured here on an upside down oil pan but actually the 12V model is more useful for heating a small, insulated aluminum tank. Use the AC model and plug in if you plan to heat a larger tank or an engine (in place of a block heater).
Veg-Oil Pad Heater for Tanks 120V 
Stick-On Pad Heater for Tanks EH-3659 250 Watt 12V DC Stick on pad heater 5 x 5 inches, heats 10-30 gallons. Pictured here on an upside down oil pan but actually the 12V model is more useful for heating a small, insulated aluminum tank. Use the AC model and plug in if you plan to heat a larger tank or an engine (in place of a block heater).