The Marine Aquarium Nitrogen Cycle

So now that we have the rock and sand in our tank, we now need to wait for the tank to “cycle.” The “cycle” is the marine aquarium nitrogen cycle and it is very important to know when setting up a reef tank.

The Marine Aquarium Nitrogen Cycle

When live rock is obtained from the ocean, or live rock farms, the harvesting process and transportation result in organisms on the rock to die. These organisms then produce waste during the decaying process. The process of this waste being converted to less toxic nitrogen compounds is called the Nitrogen Cycle. This process can take 4-6 weeks using uncured live rock. We shortened this process by buying cured live rock that has already gone through the cycling process at a retailer. This rock is usually more expensive, but it only takes roughly a week before our water chemistry is stable enough to add our first fish. We also skipped the horrible smell of decaying matter that comes with curing live rock at home for 4-6 weeks.

During the tanks initial Nitrogen Cycle, there will be elevated levels of ammonia from the dead and decaying organisms on the rock. Bacteria from the genus Nitrosomonas convert the ammonia to Nitrite. Nitrite is then converted to Nitrate, a less toxic compound, by bacteria from the genus Nitrobacter.

Initially, ammonia levels will rise and peak, followed by nitrite levels, and then they will both decrease as nitrate levels rise. Nitrates are eliminated by doing frequent water changes. The general rule for water changes are 10-20% every week or two, depending on your tanks bio-load.

Once your initial Nitrogen cycle is complete, this doesn’t mean you can run to the pet store and load up on fish and dump them in the tank. They need to be added slowly, a few at a time. Ammonia comes directly from fish waste, as well as fish food and you don’t want to overwhelm your bacteria with too much ammonia all at once. A couple of fish to start, wait a couple of weeks for the nitrogen cycle to kick in again and become stable, and then a few more.

So how do you know when your tank is cycled, or when the nitrates are high enough that you need a water change? Water testing using test kits found at your local fish store should be done daily and logged during the initial tank’s cycle. Less frequently for an established tank. More on this later.

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