A drop or rise in ph does happen, monitor it . You said your No2 is
..10
if a reef tank it should be at 0 ppm along with No3 which you didn't
mention what that was. Have these readings chanced recently? or are
you do for a water change? If it doesn't come down or is going up
it's time to do a water change then get a good reef buffer to stablize
the ph to about 8.3 for a reef.

Seachrem's Reef Buffer is what I use and I have been able to
maintain a ph of 8.3 there are other brands out there as well, kent
marine makes one and are easily found in LFS or online catalogs.

Mark