Don't get discouraged if it takes a while for the nitrites to zero out. That was the slowest part of the cycling process for us. Since the tank was cycled, we have never had another nitrite reading in twenty months.
When the ammonia zeros out, add enough ammonia to get it back to 1 or 2 ppm. That'll be enough to keep the ammonia-consuming bacteria from starving. Again, be patient for the nitrite-consuming bacteria to colonize. While they may be slow to populate your biological filter, they seem to be very hardy when they colonize. After a 7 hour power outage, we had .25 ammonia, but never had measurable amounts of nitrite. While this is slightly off subject, now when the power goes off, we add more water to the tank until the bioballs in our wet/dry filter are completely submerged. After the work of fishless cycling our tank, we don't want to lose the BB in a power outage--that would be a major catastrophe!
Review this article again. I wish I had read it when we cycled our tank. When we were at your stage of the cycling process, I kept added ammonia to 4 ppm levels. I think that delayed the cycling as it generated tons of nitrite for the nitrite-consuming bacteria to consume. http://www.aquatic-hobbyist.com/profiles/misc/fishlesscycling.html
Nice article. I am just happy I finally got the ammonia to drop. I'll give it a check when I get home and see where it is. I'm not even going to check the Nitrite level. It was off my chart. I have never seen the color so dark.
I will post and let everyone know where we are later.