those heaters have thermostats with temperature control...but you do have to watch it closely, and i'd set even a 25w heater at least 5 degrees lower than my target temp. and test it...my ebo is super stable, but it's not pinpoint accurate, it definitely varies with the size of the tank its in...ie, its in a 20g, i set it to 75 and get 79...it's in a 40g, i set 79 and get 79....etc.