Yup, I bought an XP Aqua ATO.Did you ever get to the bottom of the issue?
I can’t remember all I covered in this thread but after speaking to Tunze directly they states the 3155 is not made for tanks under roughly 50 gallons I believe is the cut off they stated. Even with adjusting the internal controller AND adding the needle valve they suggested.
the issue ended up being driven by the filter floss I was using, on top of that the inconsistent flow caused by the return pump.
the best solution I found was to use no floss or a very small piece that allows flow around it, and to separate the magnet/float away from the optical sensor.
I would not advise this ATO for any nano tank, or any AIO tank that uses a single overflow.
as a side note the Aqua XP has never had a single issue and uses a duel optical sensor.