I had 1500w on my 400g, and it was barely enough to keep it in the high 70's during the winter. But the tank was in my garage, so the room was a lot colder. Before you determine how much juice you need, you need to find out what's the coldest temp the room will hit. A tank indoors will need a lot less watts than a tank kept in an uninsulated garage.how are you guys with the huge 700 gallon tank heating them? I was looking at a 1000 watt titanum heater with controller and two receptcles to add a second 500 watt heater for my 800 gallon. Is there anything better of simplier.