quarantine tank issues

[burtonpj48]

I have been running my qt tank since may of last year. Yesterday was the first time i brought home fish, since the qt tank has been running. I checked the N03 and amonia, and ph. all the levels were good. After 24 hrs trying to qt my fish, the amonia was low but, the nitrates were off the charts, at the maximum level. After so many months of the tank running i didnt think i would have problems. How did the nitrates jump so high in one day? Has anyone ever had this happen to them?