The rule of thumb is 3 to 5 watts per gallon, if the room is really cool, around 65 degrees, you might need the higher end of that estimation, but if your room is fairly warm, say like 75 degrees, you can probably aim for the lower 3 watts per gallon. Ideally split between two different heaters so if one fails, you're less likely to freeze or cook your tank. You're probably going to need somewhere around 200 to 300 watts.