Explaining the "Cycle"

It was brought to my attention that I overlooked the fact that not everybody knows what the "cycle" is. I apologize, and I'll explain it now.

One major byproduct of organic waste is the production of ammonia (NH3 or NH4+). Ammonia is toxic to most organic life (including fish and humans). So, we need to get rid of the ammonia somehow. There are naturally occuring bacteria that consume ammonia and produce nitrites (NO2-) as a byproduct. These bacteria are called nitrosomonas. Unfortunately, nitrites are also toxic in higher concentrations. Luckily, there are also some bacteria that consume nitrites and produce nitrates (NO3-) as a byproduct. These bacteria are called nitrobacter. This biological conversion of ammonia to nitrites and then nitrites to nitrates is called "nitrification".

Nitrates can be toxic in really high concentrations (well beyond that found in a home aquarium), however that is not the real problem. Nitrates actually are food for algae. By having too much nitrate in solution, algal overgrowth (or "blooms") can occur. These algae consume dissolved oxygen, and can essentially "choke" fish to the point of death. Plus, tanks full of ugly green and brown algae are not attractive. So, we need to continue the process of removing the nitrogen. One thing I didn't mention above about nitrification is that the nitrosomonas and nitrbacter need oxygen to consume the nitrogen products. In other words, the process is aerobic. Converting nitrates to nitrogen gas (N2), which bubbles off and leaves the tank, is called "denitrification". Since air is 79% nitrogen gas anyway, this is completely benign, and ends the nitrogen cycle. Denitrification occurs best in areas of low (anoxic) to no (anaerobic) dissolved oxygen concentrations. Since the water is well mixed, the only areas in the aquarium where low dissolved oxygen concentrations occur are within the sand bed, and within the caves and small crevices of the live rock where there is not a lot of water flow. These areas are not very abundant as compared to the entire volume of water in the tank, so this process is limiting. So, how else do we get the nitrates out of the water? Water changes - that is, we remove a certain amount of water from the tank, and replace it with fresh, purified water. Hence the RO/DI of the last post. Of course, this new water must be salted and aged before going into the display tank.

So, what does all of this boil down to? What's the bottom line? In order for the ammonia to be removed from the tank, the cycle has to get started... that is, the colonies of nitrosomonas and nitrobacter need to grow to handle the bioload (read: amount of ammonia created by daily activities within the tank). By "letting the tank cycle", what I'm really saying is "giving the tank enough time for the bacterial colonies to grow large enough to handle more of a bioload". When the analytical testing reads zero ammonia and zero nitrite, then I know I can add more of a bioload (ie, more live rock or more fish). Once the colonies grow enough, the ammonia readings will again drop to zero, and I'll be able to add more, again. Eventually, I will hit a point of maximum bioload, at which point the tank is fully stocked.

Interestingly, this same nitrogen cycle is the technique we use in my job. For those of you unfamiliar, I am a sanitary engineer - meaning, I design sewage treatment plants. In those plants, we have nitrification zones, denitrification zones, etc. etc. Generally speaking, we never add chemicals, or anything, it's just a complicated system of aerating and mixing with anoxic conditions - and human waste is turned into clean water! It's funny how our hobbies sometimes tend to mimic the rest of our lives. How many computer programmers do you know who are huge gamers? Exactly...

So, Scott, was that a sufficient answer to your question?

As an update to the actual tank build - the ammonia is still present in the tank. Nitrites have started to rise, which means the nitrosomonas colony is growing. It'll probably be another week or so until I'm ready to add the second shipment of live rock, which will include all of the "clean-up crew" (CUC), which eat detritus (solid waste) and other sources of ammonia - further preventing buildup of ammonia in the tank. My CUC will consist of snails, hermit crabs, a star fish, peppermint shrimp, and a sea cucumber. Once that arrives and cycles, then the tank will be ready for fish! It's a slow process, but done right, will be so very rewarding.

This has been a very long post... sorry if it put any of you to sleep. :)