I've been cycling my new tank. I put ammonia in it the first time on 12 Dec. Nitrites and Nitrates showed up for the first time on 17 Dec. Nitrites were reading around 2 ppm and Nitrates at about 160 ppm. By 24 December Nitrites were still around 2ppm and Nitrates had fallen to about 20 ppm. 24 December was the last day I added ammonia. We were headed out of town, so I threw three little silvertip tetras in with a weekend feeder. I happened to be back in town for a doctor's appointment on 27 Dec and tested the tank. Nitrites had shot up to about 5 ppm and Nitrates back to 160 ppm. We came home today and Nitrites are at .25 ppm and Nitrates are 0 ppm. The three silvertips are all alive and seem to be doing fine. What does this mean?

Thanks!