The only reason we see a "nitrite spike" when we proceed through a normal fishless cycle procession (by normal I mean one without mature media added) is because we need to grow about 3 times as many N-Bacs as we do A-Bacs (each 1ppm of ammonia gets processed in to about 2.7ppm of nitrite by the A-Bacs, meaning that there will be triple the amount of nitrite(NO2) needing processing into nitrate(NO3) but the N-Bacs don't grow any faster than the A-Bacs, actually slower, really. So we start a tank and both the A-Bacs and N-Bacs begin slowly growing. We are feeding the A-Bacs ammonia and even though there aren't many of them, they begin producing 2.7ppm of nitrite for every 1ppm of ammonia. There are the same number of N-Bacs roughly, so they can only process about 1ppm on nitrite on in to nitrate(NO3) and meanwhile lots of nitrite(NO2) builds up. This excess is the nitrite spike and it goes way above what our liquid test kits can measure. Quite a bit later, the N-Bac population will finally get triple the size of the A-Bac population and together they will both be reducing the 5ppm of ammonia into zero ppm of ammonia and nitrite and all that will be left is lots and lots of nitrate(NO3) (unless we've water changed it out somewhere along the way.)
