I brought a 100w heater for my tank because the one i have is only a 25w. do you think a 100w heater is too much for a 35l tank?
Its fine, dont worry !
You can use a 1000 Watt heater in a teacup if you want.... the danger is that the thermostat sticks "ON" and overheats the water. (Although this should be rare these days)
You do NOT need to set it colder than you need... but I would ALWAYS trust a thermometer over the heaters settings.