Understanding the nitrogen cycle in a saltwater aquarium.

Learn how to cycle an aquarium and how to tell what state its in!

As a beginner you'll often hear that the first thing you need to do is cycle the tank before you put any fish in. Generally speaking, the cycle being referred to is the nitrogen cycle in which you allow your aquarium to build up the necessary denitrifying bacteria to make it suitable for fish. The nitrogen cycle is anything but simple, but hopefully after reading this section you'll have a basic understanding of what is going on and why you need to do it.

What is the nitrogen cycle?

Ammonia, Nitrite, and Nitrate compose the basic aspects of the nitrogen cycle you need to understand. Fish waste will initially come in the form of ammonia, which is highly toxic to animals. If there were no way to convert this toxic ammonia into something safer than it would just continue to increase eventually killing those expensive fish you painstakingly picked out. Fortunately we can utilize denitrifying bacteria to convert this toxic ammonia into nitrite, even more fortunately, you don't actually have to do anything to get this bacteria to work for you other than be patient!

How do I cycle my saltwater aquarium?

When a tank is first started there is no dentrifying bacteria to convert the toxic ammonia into nitrite. For this reason it is not safe to add a fish to a new saltwater aquarium until after the cycle is completed. Cycling the tank is easy though and provides time to carefully consider which fish you would like to add your aquarium so there's no reason to skip this important step. To get the aquarium's nitrogen cycle started all you need to do is let the aquarium run as it normally would, just without any animals. In order for the cycle to start you need to build up a small spike in ammonia for the denitrifying bacteria to feed on. In the past this was done using hardy fish which could handle the increased ammonia and nitrite levels. Presently this is considered unnecessary and generally an ill-advised idea. Aside from being cruel to the fish, it likely means you're getting fish you don't actually want and will have a difficult time catching to return your local fish store. Instead there are many alternatives which work just as well and don't leave you with an unwanted neglected fish. Live rock purchased from your local fish store for example, can provide a good source of ammonia via the decaying matter found on the rock. If the rock cured and has little to no dying matter on it than a small peice of uncooked shrimp or fish food can be used to build up the ammonia.

How do I know when my cycle is over?

Once the ammonia has risen and started to fall (how high it rises will depend on how much decaying matter there is, generally it doesn't matter as long as it starts to falls back down again) than the denitrifying bacteria are at work and converting that toxic ammonia into another form of nitrogen, nitrite. Nitrite, unfortunately is also toxic to fish and must be then converted again to make the water safe for fish. After a little over a week the nitrite level in the water will rise and then start to fall just like you saw with the ammonia level. As the nitrite level falls it's showing that a new kind of denitrifying bacteria is now present, a kind of bacteria that converts nitrite into another form of nitrogen, nitrate.

Nitrate is the final step of nitrogen cycle, nitrates are almost always present in the saltwater aquarium and as long as they are kept in a manageable level it's perfectly fine. Nitrate can reach levels of 20ppm depending which fish are kept or 5ppm depending what corals you plan to keep. Once the nitrogen cycle has reached the point where ammonia and nitrite are no longer present and that remains is nitrate (or maybe even no nitrate if you're lucky!) than your aquarium is finally safe for your first addition. It is important to add fish slowly to allow the denitrifying bacteria to increase their population in a sustainable rate to prevent the toxic forms of nitrogen from increasing. Generally you should avoid adding more than 1 fish per week if possible while keeping in mind the longer the gap between adding fish the better it will be to gauge how much your system can handle.

How do I test ammonia, nitrite, and nitrate?

There are many different test kits available for the hobbyist to test these three levels. The brand you chose doesn't usually make much difference, the most valuable test kit you can buy is the one you actually use, as in remembering to test your water is more important than the brand of test kit you use. An affordable option that can be used for measuring the ammonia, nitrite, and nitrate of the water are the common dip strips which can quickly provide results for all three levels. While we do not recommend relying on these test kits for some of your other parameters such as alkalinity or pH, they will do fine measuring how far along your cycle is coming.

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