I had a similar problem with my Nitrites awhile back.
Mine turned out to be a combination of things.
I am running a live sand bed which is not supposed to be siphoned, The flow from your return and power heads is supposed to keep much of the detrius circulating for the overflow to take out to the sump.
I had my liverock packed to tight and had minimal flow through the rock.
I could'nt get to the detrius that setteled, and like most I felt bad for the fish and over fed.
I had to rearrange my liverock and add larger powerheads, add a spray bar to my return to create turbulance on the surface for gas diffusion.
But I also had to siphone my live bed to get rid of all the junk I could'nt get before. along with a few large water changes this along with changing some of my bad habits did bring my Nitrites down.
You may have tried this already but it might help.
Good luck, This site may not be perfect but really it is a great one, we all say whats on our mind from time to time but we are human