Would it not be far more acurite to use a cheep pic at the tank with 'digital' probes and then comm lines back to main pic?

Other than that, i'm wondering why your using cap/resister solution...
Would two rods running all the way down the tank, one charged with say 5V DC and the other connected to a analogue port... as the water rises the resistance between the two rods changes and therefor the voltage on the analogue port would rise...

Just a thaught...