Hi. Ya you could probably get away with a 200W heater on a 55gallon tank. I decided to go with a 300W on my 55 though. The difference in price between a 200W and a 300W is only a few dollars so i just went with the bigger one. Should make it easier to keep the tank warm on those cold winter nights. They say you should have 3 or 4 watts per gallon of water.