First of all, your "ATO" output (the 24v Accessory port on your FMM) should not be set to the "ATO" Control Type if you own the Neptune ATK. "ATO" is intended if you are building your own DIY topoff system using either float switches or optical sensors with a 3rd-party pump.
Instead, you should use the programming automatically created by running the ATK Setup Task in Fusion. The result will be an output set to the "Advanced" Control Type, with programming that incorporates multiple layers of safeguards. Read my ATK Tutorial for a detailed explanation of the standard ATK code, what it does, and how the ATK is designed to work:
Part 3 of our series on learning how to program your Neptune Apex for automatic top-off in a saltwater aquarium.
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This tutorial also has a detailed list of troubleshooting steps to help you test your optical sensors.
As for the series of alarms you're seeing, this is a symptom of a sensor rapidly toggling on and off so quickly that the Apex doesn't have time to send the initial Alarm On message which would include the description of the device causing it before it sends the Alarm Off message.
Add this line to your EmailAlm program to force the alarm to stay active long enough to send the Alarm On notification:
Min Time 000:30 Then ON
Once you identify the source of the problem, you can investigate to see if it needs cleaning or some other issue. Also, be sure the ATK is assembled so the water does not trickle down onto the optical sensors. If it does, then you need to remove the float valve assembly and reverse the side it's connected to so it doesn't dribble water on the sensors.
