all heaters shut off when they get to temp unless the tstat is broken. The only problem will be if the heater sticks on, 300w will easily cook your fish if this happens.
You can use the 300. If you want to err on the safe side, pick up a 150 watt. It depends on what the average room temp is. I keep my room at 60-65 in the summer so I have a 250 on my 40 and I have a 400 and a 200 on my 125.