Hey Chinxican, I read earlier in the thread you were using a water filter, have you checked the water straight out of the filter for nitrites? I used to use a Pur filter for the water in my tank when I first set it up and came to find out the little bit of nitrites I couldn't get rid of were present in the water straight out of the filter..When I checked the normal tap water it was clear of any Nitrites.
Just a thought ..