I use a double float set-up similar to this.
http://autotopoff.com/products/DS1/
I actually have it right out of the water in my sump, and only use it as a secondary (and third) safeguard in case my primary float (which regulates sump level and fed by my RO/DI) fails. The primary float feed only comes on for 5 min every 6 hours via a solenoid that is connected to my Apex. If power is off, solenoid is off. The power goes through the double switch controller, and will cut power if the 2nd or 3rd float is activated.
http://autotopoff.com/Solenoids/
The double float switch will not allow power to the solenoid.
Might be a little overkill, as even my primary float has never failed. But it does give me good peace of mind.
And the whole set-up costs way less than a Tunze or most other ATOs.