[Sidius]

Thank you everyone for all your comments!!

To explain my ATO plans in more detail, I was planning to use a water reservoir with a Tunze Osmolator ATO to keep the sump topped up. The RO/DI would be plumbed with a solenoid and controlled by the apex to keep the water reservoir topped up (float switches for both low and high marks in the water reservoir). I was going to get a plumber friend who is also an aquarium junkie like myself to plumb the actual RO/DI because I admit that I know nothing about plumbing.

Would that give me the redundancy that you're referring to?

Basically the ATO keeps thet sump topped up and whenever the water levels in the water reservoir drop the Apex would trigger the RO/DI unit to fill the water reservoir back up. If the RO/DI failed it would overfill my water reservoir but it wouldn't fill my tank. The most the ATO could dump into my tank if it failed in the "on" position would be the contents of the water reservoir. I'm thinking a 20gallon tank or something would work well for a ATO water reservoir but would that be too much?

Edit: I should add that I live in a townhouse with ground level main floor so I have no basement and no closet to keep any tubs or garbage cans to store water. My wife isn't thrilled that I'm converting to a reef tank as-is so leaving anything visible to annoy her will just compound that problem lol.