I use a heater like this in my Instructable on making perfect steaks. I hook it to my themostat control project to maintain the temp.
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.
Unscrew the control knob. Mine had a catch to prevent you turning it up past 90 degrees. I had to wiggle it and use a little force. Nothing drastic though.
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.
Step 3: Remove the Plug.
This is the most difficult step. Go slow. Take your time. Be careful.
Wrap the tube in the towel. This will protect your hand if something goes wrong and you shatter the glass.
Using the pliers, grab the plug. Put one end in the hole and the other end on the edge of the plug. Slowly work the plug out. Try not to tear the rubber, or pull on the cord.
Once you get the plug most of the way out, slow down and pay attention. The heating pad will come out and you will need to get it back in. Mine had a foam insert that was wrapped around a plastic tube.
Step 4: Hack the Switch
There will be a metal strip that pushes the button on and off. I just bent the metal strip until it was always firmly pushing the switch. Even when I hit it with a heat source over 150 degrees F. (heat gun or blow dryer.)
Optionally you can remove the switch and solder the leads. I was afraid of melting the plastic heater part.
Step 5: Put It All Back Together.
This is almost as hard as getting the plug out. Go slow. Don't push too hard. Be careful of the glass tube.
Roll up the actual heating element around the foam the way it was in the first place. Gently put the heating element back in the tube.
You can lightly lube the plug with some machine oil. Gently but firmly start working the plug back into the tube. Don't worry if the last bit doesn't want to go in. It will still be waterproof if you get it most of the way.
Screw the control stem back in to finish the waterproofing.
Your heater is ready to use. To cook my steaks, I just plugged in the heater when the temperature started to drop, and unplugged it when it got high enough.
You could also make a micro-controller powered outlet and hook a thermostat of some kind to it.