nitrate help old live rock

[Chris82]

I am very conscious of feeding and honestly don't even feed enough at this point.
Doing fifty to sixty percent water changes twice a month.
After w/c nitrates go from twenty five to about ten then with in two days back up. And if I do another large WC they do same thing.
Running a skimmer.
Its just not possible IMO to have nitrates continue to increase so rapidly if they are not coming from a source.
Just don't know.......