It is very common to see water level fluctuations in the back sump/filter area on a nano aquariums. Sometimes this will be more pronounced if the main pump is in need of maintenance or the filter media is beginning to block. It is also possible that a surge or wave pump can push water towards the overflow comb into the filtration causing a fluctuation. As a result of this it is common to see the ATO sensor submerged deeper than the main sensor point and will not be a problem.