Well you've just answerd the question. Nitrite is terrible stuff and would 100% cause stress and loss of colour.. you need to bring it down, yu should have barely a trace. Have you checked your filters arn't being ristricted? Could be old tank syndrome.
Im guessing you dont you have a water testing kit? You should get one. Im supprised your SW
tank has lasted 2nhalf years without one so you must be doing something right.
If you get a testing kit youd be able to cross off some possibilities,
I know too well that rising ammonia and nitrite can be caused by blocked or restricted filters.
When was your water last tested and when was your last water change? Do you buy water from the lfs