Probably not what you want to hear, but I'd recommend getting a controller rather than relying on the internal thermostat on the aquarium heater.
I am using a fish tank heater in a water bath. When I started fermentation, I was only using the heater and had to tinker with the settings to get it where I wanted it. Half-way through, the STC1000 I bought arrived and I built the box per instructions on this site. Been much happier with the consistency of the temperature since.
The best advantage of the controller is you can control the beer temperature rather than the environment temperature (surrounding air / water).