I've had very bad luck with the 3155 failing on me, honestly the TUNZE ATO systems are not the best IMO...
One of the things I hate about the 3155, specially for a nano, anytime the power goes out it starts the pump and dumpos about a 1/3C of water into the tank. It's basically to prime the pump.
The nano I had on my biocube never did this, but I did have a float stick in the down position and dumped my entire 5G topoff container into the tank causing it to overflow.
I then checked that specific float during my weekly WC regimen.