A Brief Review of the Nitrogen Cycle

When you start a new aquarium it will go through what is called the nitrogen cycle.

The nitrogen cycle is a process which allows “good” bacteria to establish and multiple in the tank, thus breaking down the fish wastes.

Fish excrete ammonia as a waste product. Ammonia is highly toxic to fish and even small quantities in the water can kill your fish. The first bacteria that establish as part of the nitrogen cycle break down ammonia into a product called nitrites.

Nitrites are also toxic to your fish, but not nearly as toxic as ammonia is. As the nitrites start to rise in the tank the bacteria that feed on nitrites will start to multiply and consume the nitrites breaking them down into nitrates.

Nitrates are also toxic to fish, but only in very high quantities. The nitrates will continue to rise in your tank as long as you have fish and are feeding them. Water changes are used to reduce the nitrate levels.

How long does this process take?

Typically the nitrogen cycle takes between 4-6 weeks to complete. During this time it’s recommended that you take samples of your tank water to the local fish store and ask them to test it for ammonia, nitrites and nitrates. The test levels will progress as follows:

Stage 1:

Ammonia zero
Nitrites zero
Nitrates zero

Stage 2:

Ammonia starts to rise
Nitrites zero
Nitrates zero

Stage 3:
Ammonia continues to rise
Nitrites begin to rise
Nitrites zero

Stage 4:
Ammonia levels drop
Nitrites continue to rise
Nitrates begin to rise

Stage 5:
Ammonia: zero
Nitrites levels drop
Nitrates continue to rise

Stage 6:
Ammonia: Zero
Nitrites: Zero
Nitrates continue slow rise

Tips to reduce the time of the nitrogen cycle.

The easiest way to reduce or eliminate the nitrogen cycle is to use a filter from an established tank. This filter already has the bacteria needed to breakdown ammonia and nitrites. By using an established filter you essentially bypass the nitrogen cycle.

Another easy way to shorten the cycle is to use “seed” gravel from an established tank. Simply take a few cups of gravel from an established tank and add it directly into the new tank. This gravel will contain a starter of the bacteria needed. If you don’t want to add the gravel to the tank because it is a different color/stone than your gravel you can simply put it into a nylon stocking or cheese cloth “bag” and drop it into the tank. Using this method can cut your cycle time by 50%.

Finally there are live cultures of bacteria such as Bio-Spira (available at http://www.fishstoretn.com ) which will seed your tank with the bacteria needed. This product can virtually eliminate the cycling process.

Stocking your tank during this period.

During the cycling process you want to stock your tank very lightly. This is because if you have too many fish the ammonia will build up faster than the bacteria can break it down and your fish will die. I recommend only 1 or 2 hardy fish such as mollies. You want a fish that can tolerate the fluctuating ammonia and nitrite levels during the cycling process. There are many fish which should be avoided during this process including all catfish and any scaleless fish such as loaches.

Feeding during the cycling process.

I recommend that you feed your fish only once per day, and only as much as they will eat in 5-10 seconds. You should net out any food left over after feeding to prevent it from breaking down into ammonia. Once the cycling process completes you can double the feedings to twice a day.

Water changes during the cycling process.

If done correctly your tank shouldn’t need any water changes during the cycling process. However should you find that your ammonia or nitrite levels are dangerously high during the cycling process you should do an immediate 25-50% water change to lower those levels. Water changes during the cycling process can extend the time it takes for your tank to cycle. Once the tank has completed cycling you should start your weekly 10-25% water changes to lower the nitrates in the tank.