Alot of what was discussed is doable. Use the light+sensor+bouy sensor type thing. Pretty lights, a resonably accurate level sensing and you can adapt to the initial schematic. Using a phototransistor you can control directly the output. It'd be like using it as a relay.
Or, use the LDR on the bottom of the reservoir and a few leds on top, and you'd have a variable resistance (use a wheastone brige for compensating) . As the level fluctuates , so does the light reaching the bottom. But that depends alot on the transparency of the fluid and exterior light (which if it stays inside the case, shouldnt be a problem).
Both of these look to me like the real "easy" ones.
