I have a 200 watt heater in a 65 gallon tank set at 80. It's a freshwater tank, not that it matters...
I keep my house at 68 to 70 in the Winter and the heater I have in the tank won't get past 77 and mostly stays at about 75. I'm going to add another 200 watt heater. I'm getting one that will automatically turn off at 93.