I wouldn't try anything as drastic as carbon dosing on a new un-established tank. My only guess for a 160ppm NO3 reading on day 25 and a 20ppm NO3 reading on day 27 without a drastic water change in between is an error in testing on one or both of those days. I would suggest seeing if you can get another test kit and confirm your latest reading. If you think your cycle is done, ghost feed your tank for a few days and see if that has any effect on your ammonia levels. If there's no ammonia spike and the nitrate levels slowly start coming up a bit, do a water change and add a bit of a clean up crew and a fish or two and slowly build up that bioload.
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