I run into a similar issue on my tank. In a nutshell, my water level varies by a good 2" between when the return pump is on v. when it is off. I think this is because of how much vertical space there is in my overflow between the main drain and the emergency drain. In any event, we both have big swings in our water levels. The issue I have is that when the return pump is off, my optical eye level sensor is fully submerged. In this fully submerged state, it reads as "too low" and activates the pump. I also have an emergency, mechanical float switch, but if I mount it low enough to prevent this, the alarm goes off anytime I'm feeding the fish. Point being, there's a no-man's land dead spot between the two sensors, so in some instances, I have only one backup.. the mechanical float. At the moment, when I turn the return pump back on, my ATO will kick in briefly, until the return pump drops the water level to a point where the optical eye isn't submerged. I wonder if your issue isn't somehow similar? I plan to resolve it by putting a longer stand-pipe in my overflow so less water drains into the sump when the pump is off.:mad: My ATO tried to overfill my tank ! One day in ! (This is why I am writing this at 3:00am. I suspect it was sensor placement but I did discover a design flaw! (They all have them)
I woke up to nature’s call and could hear a strange sound in my tank. It was the ato pump running dry in my reservoir. The light on the sensor was blinking blue. The water was well above the ATO sensor. Fortunately I only had a little over a gallon in the reservoir. This was less than the safety volume of the tank so no wet floor.
My theory :
Micro bubbles tripped the sensor while it was under water. The Controller did not yet setup a safety timer and ran the reservoir out.
The possible design flaw:
The pumps safety is that the pump will not run longer than 3x the time it took for the first run. The issue: if it overfills on the first run there is no safety! That is the design flaw.
Other info:
I could see small bubbles at the sensor where I put it. They didn’t seem to interfere the first day.
My water level was a little high so I shut off the ATO and moved the sensor lower (putting It slightly underwater ) . I then turned the ATO back on and it booted and indicated it was ok with a solid blue light.
I went to bed ....... woke up to nature’s call and you know the rest.
I suspect micro-bubbles may have triggered the ato to fill. Since it was underwater it just ran and ran because the safety never got set since I power cycled and this was the first run.
I will check with the vender in the morning.
Changes to my design
Installing the mechanical float valve on the fill line ASAP.
I may add an external safety as well.
Aqua Auto AutoAqua Smart Level Security unit
https://www.marinedepot.com/search?Query=smart+level+security&oq=Smart+level+security
Before you ask if I regret returning the nano . I did at first but after thinking about it. The nano could have also failed on for a similar reason (meaning sensor error). It’s timer is preset so it would have shutdown after 1.4 min. So in this scenario it would have performed better. However it is still using a relay and can get stuck in the on position ignoring the timer with the same results. So six of one 1/2 dozen of another.
UPDATE: I worked with coralvue tech support and it is not super clear about why it did what it did.
Three theory's are:
1) Debris or microbubbles caused a false trigger
2) The water by the exit of the protein skimmer was too turbulent for this type of sensor. Causing a false trigger
3) flakey\defective sensor that could be affected by just about anything.
All three failed on the first run since power on thus bypassing the safety since it never set the base time. This caused the pump run until I shut it off. (they really should program in a default time of 10 min). Just not a super common situation.
A simple move of the sensor would knock out two of the three. I moved it to the other side of the sump (over the circulation pump).
Original sensor location
New sensor location