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Your schematic does not show the location of sensing electrodes in water tank.
Depnding on mineral content in water, its conductivity varies. Distilled and de-ionized water is highly resistive while sea water would rather short the electric circuit.
Size and shape of sening electrodes is important as well as their distance in water tank. Take a DVM and measure electrode voltages, then adjust the circuit to operate.
I have had better success at this, by using a power mosfet to operate a relay.
The gate is held at source potential via a high value resistor shunted with a zener diode (to protect the gate) and the relay stays off.
When the probe touches water, even the most minute current will turn the mosfet on.
Its then just a case of arranging a suitable potential between source of the mosfet and the water, either ac or dc will work.