I had a 200W in my 55-gallon for a while. The problem was, the temp varied alot, it got a lot warmer during the day then at night. This one did not have a thermostat built in it.
So, I purchased another 200W with a thermostat. I had the first heater at 3/4 power which kept the water at 70-degrees at night, which we keep it pretty cool in the house. I added the other 200W with the thermostat set at 80-degrees.
For the past 6 days, my tank temps have not fluctuated in the least, a steady 81-degrees day and night.
The nice thing is, if one happens to go out, the temp won't drop below 70. Now I just cross my fingers that the thermostat doesn't get stuck open.
1 Yellow Tail Damsel
1 3-inch Domino
1 Yellow Tang
1 Picasso Trigger
1 Green Wrasse
1 Tomato Clown
1 Coral Beauty