Once the "nitrite spike" stage of fishless cycling breaks and you finally see some zero ppm readings for nitrite(NO2) 24 hours after ammonia was dosed, you are in to the 3rd stage of fishless cycling. I believe you have already done this, you ease your dosing back up to the 4-5ppm level and begin to do some 12 hour testing in addition to the confirmation tests at 24 hours if needed. Usually you can tell your progress through the 3rd stage by how much nitrite still shows up at the 12-hour (12 hours after dosing) nitrite test, whereas usually the 24-hour nitrite test will be zero ppm or a trace.
Often the main problem we see in the 3rd stage is that either nitrite or ammonia or both will exhibit some "sticking" at trace levels (for ammonia it even feels like it has "backtracked" from what it was accomplishing previously) and it will just do it for days on end, not allowing you to see those 12-hour double-zero (double meaning both ammonia and nitrite) readings that you want to be able to start your qualifying week or get through it.
Some felt we should wait for double-zeros at 8 hours after dosing and some felt that 18 hours was ok but the reason we settled on 12 hours was that virtually all the fishless cycles that completed with that kind of qualification week showed no mini-spiking from the biofilter after the first fish introduction. This is an advantage as fish are most vulnerable after transport and acclimation and introducing them to an environment where the biofilter is rock solid rather than fluctuating at its job increases your success.
~~waterdrop~~
