Basics Of The Nitrogen Cycle In Saltwater Aquariums

When browsing reef forums it is difficult to steer clear of questions from newbie's who rushed into their first tank without understanding some of the basics. Being guilty of that myself I understand why it happens, it is exciting and the tanks we see at the fish store are amazing and after all..... What's the worst that can happen right?

Well, the worst that can happen is that living beings die because of ignorance. Second to that is the fact that saltwater livestock is expensive and if the newbie reefer continuously loses critters he or she will quickly lose interest or patience.

I personally feel that a person beginning a saltwater tank needs a grasp of the nitrogen cycle.
Without this basic knowledge of reef chemistry the tank is doomed. It is not necessary to be a chemist to understand this process. Personally I claimed an embarrassingly low score back in school when attempting to fudge my way through chemistry. If I can make some sense of it, so can everyone else.

I don't believe it is very difficult to come to a point of understanding on this topic. Lets go ahead and do what we can to fill that empty space!

In a reef tank, or even a fish only tank there are a lot of chemicals and gasses interacting.
One of these interactions, everyone touching saltwater will experience.
It is the "magic" of turning ammonia into nitrogen gas. Fish feces and uneaten foods will break down and a product of this is ammonia. Ammonia as I am sure you know is toxic and naturally we don't want this in our aquariums. Fortunately saltwater aquariums are one of the last strongholds of bacteria our moms have not annihilated yet. We could not keep fishes and corals in our homes if it had not been for these bacteria.

Some of the bacteria in our reefs convert ammonia (NH3) into nitrite (N02-).
Now I do not know much about nitrite, but through reading about the cycle I have learned that it is almost as bad as ammonia and fortunately we also have bacteria to get rid of this nitrite. The next step turns nitrite (N02-) into nitrate (NO3-) which while not great for our reefs is at least not as harmful as nitrite. In fact some life such as clams and macro algae need nitrates. At this point the cycle is somewhat complete, the last step is to get rid of the nitrates. This can be done in a couple of ways such as water changes, harvesting macro algae in a refugium or by using a deep sand bed.
I use a refugium and do monthly water changes. The macro algae will absorb nitrates and when you prune the algae and discard of it you will manually take out nutrients (nitrates among them) the macro algae have absorbed.
A deep sand bed will form anaerobic zones where nitrate (NO3-) turns into nitrogen (N2) and then this gas will evaporate out of the tank.

When starting a new tank we add sand, live rock and saltwater. This alone seems ideal for saltwater fishes right!
Well, what we want to do now is wait. We want to wait for a couple of weeks give or take....
During this time the tank will cycle, this means that the live rock you added, or a cocktail shrimp as some people use will start to break down and cause a spike in ammonia, now because this is a newly set up aquarium we do not have much of the bacteria needed to convert the ammonia into nitrite, so we wait while the bacteria multiply. When you test for ammonia during this time the readings will likely be off the scale, after a while ammonia readings will reach zero while nitrite readings will shoot up.
Again this will take a couple of days while the bacteria establishes itself.
After this the tank will have a nitrate spike and at this point the tank is cycled since we do not consider nitrate to be as harmful.

This image was copied from the book The Saltwater Aquarium Handbook by George Blasiola