It is a good idea to rely on two smaller heaters rather than one huge one, because the huge one can get "stuck" and never turn off. If one small one gets stuck, the results are not as catastropic, because the other turns off. That way if a malfunction occurs, the tank is not heated as quickly and not as hot. It learned this the hard way last week, and nearly had a disaster. In other words, use two 150 watt heaters instead of a 300 watt, or whatever applies in your case.

