Visi Therm, all the way, here. How much wattage you need will depend on your area climate and what temperature you keep your rooms at.
I like 3-5 watts per gallon as a general thumb rule, and split the wattage between two heaters on the larger tanks.
I like 3-5 watts per gallon as a general thumb rule, and split the wattage between two heaters on the larger tanks.