It depends on few things:
- The difference between the target temp and the ambient temp
- Thickness of the glass
- Whether the tank covered or not.
- The sum of heat produced by equipment
The first and last one are the most important. Equipment and lights can add from 5 degrees to more than 15 degrees. The larger the difference of the target temp and the ambient temp, the bigger the heater you will need.
2 800W should be minimum, unless you are located in the south. If you are in the north, you may need bigger heaters.