Here is a thought,
Rather than doing a really trick sensor (with all the associated fail safe, fault tolerant, etc circuitry) let us try a rather basic…electro-mechanical sensor.
Within the res add a small vertical chamber that is open at the bottom and top
Within this chamber place a small buoyant float assembly that has a magnet attached to it. (Could be encased in plastic if the 'fear' of metal in the water upsets you)
On the outside of the res attach small reed switches at various levels
Then use the reed switches to trigger whatever type of level monitoring you wish to use.
wj
