I would probably use a microcontroller for that kind of thing because it would be easy to modify the software for the purpose of adding features or improving the design etc. However, you could easily complete that project without a microcontroller and it sounds like this would be your best bet.
You'll need some form of temperature sensor for the front end of the project, and a sensor which converts temperature to a voltage would be ideal (there's loads of these on the market). You wouldn't need a linear sensor provided you're willing to add in some form of calibration so that the preset can be setup, although there are linear sensors out there which will convert a temperature, in degrees C or F to a voltage in a linear fashion. You'll want a comparator for the temperature sensor output, which will compare the temperature of the tank with your preset (a bias voltage on the other pin) and then make a decision at the output as to whether the tank is hot enough or not. Convert the output to a logic level so that, for example, not hot enough= logic0 and hot enough=logic1.
A 555 timer could monitor a time period and, via some simple logic control, a circuit could be designed which would check the temperature of the tanks compared to the preset at certain time periods. If, after one time period (the length of time of which would be set by the 555 timer), the tank was not hot enough, a second heater could be turned on. If, after 2 time periods the tank wasn't hot enough then a third heater could be turned on etc.
The tank heater control could simply be a decade counter which is incremented if the tank isn't hot enough, or even a simple binary counter with some decoding logic on the output.
I'm not going to design your project for you, because clearly that's your job - but I hope this has given you a few ideas to work with. I haven't given you a solution, but there are a few ideas there which should allow you to come to a solution yourself.
Let us know how you get on and, once you're a bit further into the design process maybe you can ask some more specific questions about how to tackle certain problems you're having.
Brian