04-14-2010, 08:36 AM
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Honestly Kevin, we rarely even discuss the cycle in a saltwater aquarium. If you are setting up a tank with live sand and dry/live rock as you describe, you will probably not even see a cycle. The rock and sand already contain all the necessary bacteria you need to process wastes. At this point i should point out that there are 2 conditions to that make this work. First, the live rock should be cured ahead of time, either at the pet shop or at home. Second, I assume you are using a protein skimmer.
When you think about a saltwater tank it is more important to observe the aquarium for signs of maturity. Having a mature system is a much more important consideration, as mature aquariums are much more stable and capable of a much wider variety of life.
When we talk of a mature aquarium, we are looking for a number of things. First, within a couple of weeks you will see a diatom bloom. The diatom will appear almost overnight, looking like a rust colored algae covering your rock and often sand. It will go away on its own almost as quickly.
Next you should see coraline algae begin to spread across the glass and rocks. Coraline algae is a pink/purple colored algae that is highly beneficial for your system. It is sign that the overall environment is beginning to stabilize, especially alkalinity and calcium levels. By the way, you should begin testing and adjusting both alkalinity and calcium when you see the diatom bloom appear.
You should also see microfauna begin to spread rapidly, both in your substrate and on the glass. Look closely for copepods and amphipods, which will appear as very small tics and worms, that scurry about in the sand and on the glass. These can be hard to see.... it takes a trained eye so be patient. Once you get used to looking for them you will see hundreds of these critters. A thriving population of copepods and amphipods is very beneficial to the system as a whole, so it pays to not add livestock until you see this. Generally, when the diatom bloom has passed, the microfauna populations will be thriving.
Finally, I would use the Nitrate test as a key indicator as well. In most systems you will see the nitrate peak and then begin to drop. It will eventually level out near zero, or somewhere under 10ppm. Good skimming, good aquarium husbandry, and a proper sand bed depth should keep nitrates at these low levels.
Hopefully this gives you some different ideas to think about, because ammonia and nitrite levels are really an afterthought in the world of marine aquariums.