I encapsulated my probe in some aquarium tubing and sealed it with aquarium calc. This will control 10KW. It is pretty robust.

I used 2 1000 watt stock tank heaters. They cost about $25 each. Be sure to get the ones for making the water HOT. Most shut off at around 40F and are just there to stop freezing.

Aquarium heaters are not up to the task long term. They foul easily and require frequent cleaning. Farm stuff is designed to use it for years, then chuck it out.