Out of the box these heaters have a limit of about 90 degrees F. That's good for live fish, but I wanted to be able to hold water in a cooler at up to 150 degrees F. A computer controller immersion heater is $200+. A thermal probe and an aquarium heater are $60 or less.
This Instructable will show you how I did it.
1. Always use your head. House current, water and glass are involved. Lots of ways to get hurt.
2. Always keep stuff unplugged when you are working on it.
3. Read the warnings on the heater before you void the warranty.
4. Never dunk a hot heater into water. The glass will shatter.
5. Always use a GFCI outlet.
Step 1: Gather Materials
You won't need much.
1. An aquarium heater.* Mine is 30 watt and works easily in 10 gallons+ of water.
2. A few towels to hold the tube or maybe soak up the blood if you get too rough.
3. A pair of needle nose pliers.
A soldering Iron and solder.
*Get a new one! If you can see coils of wire inside, it's the old style. I would not mess with that kind.
Step 2: Remove the control knob.
You can remove and discard the collar that was making it hard to unscrew the control knob. It will just be in the way from now on.