sapieus:
The problem is, the current can't be more than 0.5mV between the sticks (alive animals in the water...)
Don't stick electrodes in the water, they WILL corrode and lose their function. Use a proper level switch (float switch) instead, and you not only have a sensor that lasts, there's also no electrical connection to the water.
Your exposed electrodes will also cause problems with sensors such as EC and pH probes, which are common for water quality monitoring (due to ground loops).