Apex can't tell what the Tunze ATO is doing directly as there is no interface to it to connect to. I have Set ON as well -- IMHO what you have to decide for your self is if you want Fallback ON or OFF should the Apex loose contact with your EB. Comes down to how well you trust your ATO and to what degree you may desire to potentially override your ATO unit should something else fail in your system. In my case, I have added logic that if my leak detector senses water on the floor I turn off my ATO (along with other external pumps) just in case until I go see what is going on. I have also added independent high/low float sensors in my sump, and if the sump is too high, I also turn off my ATO, again, just in case until I can determine the problem. I trust my Tunze ATO, but having secondary systems makes me all that much happier. As such, I have Fallback OFF for my Tunze ATO, since alerts will notify me if power goes out or other unusual conditions happen, and it's not the end of the earth for my ATO to not do its thing for a short while until power comes back on or I check and resolve what else is going on in my tank.