if the sump water level is dropping, it would suggest that the siphon has failed and you had better stop the return pump fast..
I think this would be a bad Idea unless you also have a Float switch to replace Evaporated water.. I would suggest just drilling your return line just below the waterline in your tank.. Or putting on a flap valve so that water can only flow 1 direction.. That would be the safest method for both your livestock and your floor.. FWIW
most siphoning happens when the power is already off therefor your return would already be off.. The flooding usually happens at the sump level do it it being lower than the tank..