if your tank is cycled you shouldnt ever have nitrites. The bacteria is supposed to be converting ammonia to nitrite and then to nitrate which is less lethal for fish and corals.
How did you cycle and when? What is your fish and coral list like? You might have too much bio load for your bacteria.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.