Quantised,
I like the twisted bar idea, but heres what im going with...
Ultra sonic sensors that measure the distance from the water and compare them with a preset distance (the empty distance)
I will simply drill a hole at the top, and mount my device on.
No holes in the side, no contact with the water, no corrosion, rust, engineering issues..
and it will finally teach me how to use these sensors!
I will encorperate a temperature sensor in the tank to tell the PIC to change a variable.
(the variable being time taken for sound to travel through air)
So when the µC is making the calculations, it takes air temp into consideration.
In summer the tanks internal air will rise dramatically on hot days with sun on it, so using the temp sensor will keep the readings accruate.