Tank Heaters - question

[Coralgurl]

My 55 gl sw was set up in April this year, therefore this is the first winter we are going through. The tank is set up in our dining room/kitchen area and about 1-1 1/2 feet away from the window. I keep the tank temp around 80, at night it would drop to 78/79 and during the day, would go to 82 on really hot days in the summer. I've noticed since its gotten colder outside, I couldn't get the tank above 77. I went and bought another heater yesterday and this morning with the lights out, the tank was at 79.

The heaters are in the tank, I don't have a sump. How many heaters do most reefers run, obviously depending on the size of the tank. I've got 2 fluval heaters rated for 65 gl in the tank now.