Ycan play it by ear if you want to. Try one 200 or 250 watt heater on top of your 100 watt heater and let it run for a day or two. Remember to check the temperatur around the day. In the morning, noon, evening, night, etc. If the temperature remains constant, good news, you should be alright. Considering its also winter time, you'll have a good idea how stable your tank is even during a cold night. If you find it doesn't tend to stay very stable, add another 200 or 250 watt heater, and that should for sure keep your tank at a stable temeperature.
From personal experience:
My 66 gallon malawi tank used to only have one 200 watt heater on it. I set this tank up during the summer time and the heater had no problems in keeping the tank at a constant 25 degrees C. However, as winter came around, I noticed that the heater was constantly on, and that during the night, temps dropped to as far as 22 degrees C. Too far for my comfort, so I got myself another 150 watt heater. Temps have never flucuated much since then.
When positioning your heater, you can do a couple of things... position your heater where water flows the most. Other ppl position the heater horizontally along the substrate. Either way will work.
Some brands I would suggest are Tronic heaters (made by Hagen) and Ebo-Jager heaters. I own both, and have nothing bad to say about any of them. From what I hear, many other aquarists have similar experiences.
HTH
-Richer
