For the heater you may want to not refer to it as a resistor but rather a Heating Element. A Google of "submersible heater element" which will bring up a few dozen examples in assorted sizes and voltages. Next to control the process you write some temperature control (process control) software for your Arduino. Your Arduino will not directly drive a relay coil so you want a transistor in the mix to drive the relay coil calling for heat. You also need a sensor to measure temperature, like just for example a Type K thermocouple and a MAX6675 breakout board to read the thermocouple and pass the temperature data along to your Arduino for your process control.
Anyway forget about using the term resistor as you will do much better using "heating element" and for your project "submersible heating element".
Here is an example of a 1,000 watt 120 VAC element. Depending on how much heat and how fast you could parallel a few. You could also add a mixer or bubbler to prevent stratification of temperature layers in your tank. The better you define exactly what you want/need the better the help you will get.
Ron
There are several ways to go about all of this.