For most first or second time cyclers the most common cause if problems is from nitrites going to high. This happens because too much ammonia is going in. People either add too much or too often or even both. And the tank appears to handle this ammonia. But it is what one cannot see that us going to cause the cycle to stall or even crash- high nitrite.
Most filks are using API kits or ones which read the same scale but which top out anywhere from 5 ppm to 10 ppm. The problem is that the level on such tests where nitrite is a real problem would be just over 16 ppm. Allowing for test kit error etc i like to back tat red line down to 15 ppm. But how can you know if you are near 15 ppm if you kits doesnt go over 5 or 10? There are ways to due diluted testing, but this makes things more complicated for new fish keepers.
As a result, I designed the method now in place here to prevent this. The amounts and timings for ammonia additions, when followed, make it impossible for one to have nitrite issues even when they might be off the scale of one;s test kit. One of the methods intended to prevent such problems is the need for 2 consecutive 0 ammonia readings. At that point of your cycle it means somewhere between 2 and 3 days must pass before the ammonia is added. That is that much more time that the nitrite colonies have to reproduce and grow which means they are converting nitrite during that time and at an increasing rate. The two consecutive 0 readings are another form of safety net in terms of preventing issues,
The thing that many folks do not know about is how hardy the bacteria are when they establish in their bio-film. The can go without ammonia for some time without suffereing. But, in those cases where there might be some loss of the ammonia bacs because the nitrite ones are dragging, the final additions near the end of the cycle done at 3 ppm levels will very rapidly get the ammonia bacteria up to full strength again. We are talking a day or two at the most.
The hardest parts of the cycle are the earlier ones. That is because the rate at which a colony expands accelerates as it progresses, it takes much longer for the 1st ppm of 3 to be processed than it does for the last ppm. The reasons is simple, there are many more bacteria to consume that last ppm than there were the first ppm. That is why the first 3 ppm addition takes longer to drop to 0 than the second one and the end of cycle additions drop even faster with it finally taking 24 hours or less to hit 0.
The nice part of all this is the bio-chemistry involved. If one could add ammonia but have it 100% converted to nitrite for which there was not a single bacteria present to handle, exactly how much nitrite than can make. 1 ppm of ammonia will become 2.55 ppm of nitrite (on API type kits), so adding 3 ppm of ammonia can only create a maximum of 7.65 ppm. Given there are two ammonia doses to 3 ppm, that might make a max of 15.3 ppm of nitrite. But because the second addition can not be made unless some nitrites are present and at a level easy to detect, there odds are very good there some processing capacity for nitrite at work. This means there is almost no chance that nitrite can reach danger levels that one cannot see when the snack dose is added. Think of it as a bridge between the second 3 ppm addition and the next one that happens near the final days of the cycle. And that is why I used the term snack. it is to insure the ammonia bacs do not go overly long without any ammonia. However, it also means the snack addition will not create a nitrite problem.
The real challenge in creating a cycling regimen that will pretty much work for anybody using it is that no two tanks are exactly alike. Different tap water, different tank contents, different mistakes folks may make all factor into it. So the method has some tolerances built into it which are there to prevent the serious type problems one used to see on the site when ammonia dosing was done at much higher levels and usually more often.
I hope this has made things a bit clearer and easier to understand.
