There are several benefits to doing a fishless cycle. The most obvious and single most important reason is obvious, no fish can be harmed. While that in itself is enough of a reason for going fishless, there are other reasons. The next biggest benefit is that it allows one to stock 100% all at once, This has the additional benefit of making it unnecessary to use a quarantine process. The tank itself serves that purpose since all the fish go in at once. If you decide to stock gradually over time, using a quarantine tank is strongly recommended.
The other side of the process is that over time without a full load, the bacterial colonies will reduce in size down to the actual bioload of a tank. While the bacteria are hardy and have strategies for surviving hard times, lowering ammonia is not one of these. They will actually do better in this respect in times of no ammonia as opposed to too little ammonia. Since there is no formula or detection method one can use to determine such a loss of processing capacity, the first hint that if this will be ammonia and nitrite coming back,
If one decides to try and stock gradually as part of a fishless cyle, then it becomes a waste of time and testing to shoot for 3 ppm. This amount is intended to insure a tank is safe for a full fish load to go in all at once (or in a fairly short time period). If all one is going to do is put in something like 1/3 of the stocking (based on bio-load) then all you need to do is cycle the tank to 1 or 1.5 ppm. And this will go faster than doing it for 3 ppm.
The problem is one is now turning a fishless cycle into a partial fish in cycle. If sufficient time has elapsed between the first and second additions, the bacteria at work will have begun to downsize towards supporting that level of stocking. Now when one adds the next fish you may need to deal with cycling issues all over again. One needs to create a plan for adding the new bio-load to the tank in a manner that will cause the fewest cycling related issues.
One way to look at setting up a new tank and having it moved from that point to where it is completely settled in is that the important heavy lifting mostly happens in the first 6 to 8 weeks where cycling is the over riding issue. However, just because we are focused on the autotrophic nitrifying bacteria doesn't mean most of the other types that are part of a healthy tank are not also multiplying and becoming established. The bio-film in which the nitrifying bacteria live and use to attach to surfaces contains a host of other bacteria and all of them work in concert in the bio-film. The reason we concentrate on the nitrifyers is, without these, a tank can not survive very long.
In the long run a stable healthy tank depends on more than having established the invisible world of microorganisms it contains. Regular maintenance and care are required to keep our closed ecosystems functioning smoothly. No matter how well established any tank might be, neglect will quickly undo things.
Now to circle back to the original Q. When adding fish gradually it is not the number if fish that must be considered but rather the amount of waste they will create. Bear in mind that fish poop and uneaten food break down into ammonia but that fish also exhale ammonia. The goal in gradual stocking is to work to control how the bio-load is increased. A rule of thumb is to shoot for something in the range of 20%-25% increments. This will keep each ammonia and nitrite spikes to a minimum and to make then manageable. In this respect think in terms of body mass. More body mass = more ammonia creation potential.
