Here is the explanation from a local guru for the 12 weeks cycle:
I think there is confusion, because different people define cycling in different ways. Everybody agrees that cycling is about establishing the bacterial base for biological filtration. But some hobbyists (sometimes intentionally, but more often unintentionally) cycle just enough to create bacteria for nitrosification (the conversion of ammonia into nitrites) and for nitrification (the conversion of nitrites into nitrates). This may suffice for a fish-only tank, or for some other nitrate-tolerant livestock. But for most reef tanks, cycling should extend all the way into creating bacterial colonies for denitrification (the conversion of nitrates into harmless nitrogen gas).
What I always suggest to any potential reef-aquarist is to do a heavy-duty cycling method all the way to denitrification, that has the following characteristics:
1) intentional creation of very high nitrate levels (40-50ppm or more), through the use of rotting shrimp (I actually go through two cycles of that)
2) during cycling, no intervention or support from the aquarist (especially, no water change!!!) Let your system "learn" and evolve to manage nitrates and to build up its denitrifying bacterial base (especially if you have a DSB in your tank or sump or in an RDSB).
3) during the later part of cycling, allow the shifting nutrient imbalance (especially the high nitrates) to induce successive growths of various pest infestations (diatoms, cyanobacteria, hair algae, etc). Leave them alone, with minimal aquarist intervention or support, until a biological balance is reached (and these infestations will naturally disappear).
4) eventually, your denitrification capability will be established, and nitrate levels in your tank will stabilize to an acceptable level (not necessarily zero: acceptable levels of nitrates will be defined by the type of livestock you wish to sustain)
5) the above process should take maybe 10-12 weeks.
If you follow my process above, then you should not turn on your lights, until the mid-portion of the second shrimp cycle when nitrites have become zero, nitrates are very high (say 40ppm), and you are now hoping to induce the first growth of the successive pest infestations. Remember these pests (cyano, etc) are all photosynthetic (in different ways though), and their growth will be encouraged by the presence of light.
After this 10-12 week period of cycling, you can now assume that your tank's denitrification capacity is at the highest level that you can achieve for your configuration. It is at this point that you may want to further augment this, by using other "natural nitrate reduction" or NNR techniques (such as nutrient-export through macro-algae, or nutrient-scrubbing through microalgae-based ATS). But that is already separate from the cycling process. Again, I stress that other NNR techniques should be started AFTER cycling (you let cycling establish the maximum baseline denitrification capability for your tank, and later you try to improve that baseline with other NNR techniques). <<And>>
Now this is where the real problem arises. Unfortunately, many other web sites and many local "experts" have a different definition and/or process for cycling. Especially for fish-only tanks, cycling is often defined as ending whenever ammonia and nitrites reach zero (and the resulting high nitrates are manually handled by doing water changes). Anyway, such a "limited" process could be finished in 2-4 weeks. Actually, this is partially valid for fish-only tanks, because the only real interest for FO's is developing the bacterial bases for nitrosification and nitrification (they don't care for denitrification, since most fish will survive with high nitrates anyway). Actually, many web sites "lifted" these processes from "fish-only' marine tank reference books in the late 1980's and early 1990's!
My concern is that many posts both here and in other local sites usually reference this "shortened" process. They do not understand that they are essentially depriving their tanks of the chance to build up a strong denitrification base. And this is the reason why many of these tanks do not perform well as reef tanks (due to an inadequate bacterial foundation for denitrification). Typically, such tanks "crash" or behave problematically whenever there are major "incidents" of biological imbalance. This is IMHO another reason why many of these improperly-cycled tanks seem to develop hair-algae problems later in their lives.
As I posted much earlier in this thread, there are different ways of starting the cycling process (in reality, those different ways correspond to different target levels of nitrate-creation, as well as different "speeds" in doing so). There are many trade-offs, as always. At the risk of oversimplifying, the choice of process will define the denitrification capability that your tank will have at the end of the cycling process. And in many aspects, this will dictate the stability and resilience of your reef tank. And again, let me re-state that this aspect is more critical for smaller (nano) reef tanks. My suggested cycling process is designed to create the maximum denitrification capability. If that is not desired, then other methods can be explored.
Again, the above post represents my opinion, even if they currently are the "best practices" that I actually use personally for my tanks. But I cannot guarantee that they will work well for you, especially of you do not understand the objectives of my cycling process and the science behind my cycling method. The worst you can do is to "mix-and-match" different steps from different cycling methods, as their differing objectives (and the possibly different science behind them) could have inherent conflicts that will only result in a lousy compromise (and a problematic tank).
Hope that helps! Smile
