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Nitrogen Cycle

Starting a new tank and the nitrogen cycle.

By David A. Lass |

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The best way to set up a tropical fish aquarium is to stock it heavily with aquatic plants, and then add some algae-eating shrimp. Take readings for ammonia, nitrite and nitrate every day, and put in a pinch of flake food once a day for a week. Then start adding some fish, starting out with small, peaceful and hardy folks like zebra danios. As the aquarium matures, the nitrogen cycle will get going. You should get some test strips so you can test every day or two for ammonia, nitrite and nitrate.

freshwater shrimp tank. Photo by Konstantin Matern/Flickr

The nitrogen cycle is the single most important thing to understand about how we are able to keep tropical fish in glass cages. A good way to visualize the start-up of the nitrogen cycle is to think of the McDonald's golden "m" arch, but with three arches, each one of which overlaps the next one.

The first arch is ammonia. As the few tropical fish and shrimp in the aquarium give off waste (ammonia), the arch for ammonia starts going up. When the ammonia arch is near its top, the middle arch will start -- this is nitrite. Over time you will notice that ammonia starts coming down as nitrite goes up. Both ammonia and nitrite are very poisonous to tropical fish and invertebrates, ammonia being the worst, and especially if the pH is above 7.0. Soon the middle arch (nitrite) peaks, and the third arch starts on its way up -- this is nitrate. Nitrate is not that harmful to tropical fish, and it can be removed from the water by having lots of fast growing aquatic  plants in the aquarium, and doing a regular water change of 25 percent once a week.

When an aquarium has "cycled" that means that the various beneficial bacteria that work the magic of the nitrogen cycle — that change ammonia into nitrite, and then nitrite into nitrate — have grown to the proper quantities needed for the aquarium. When the aquarium has cycled you can safely add some more fish — a few at a time. A mature aquarium will be able to keep the nitrogen cycle going since the good bacteria will increase in numbers as you add more tropical fish.

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Reader Comments

Kevin    Oklahoma City, OK

7/29/2015 6:10:21 AM

You forgot to mention how important alkalinity is when cycling a freshwater tank, this is something I learned recently the hard way. If alkalinity is low, the tank wont cycle. I always use FritzZyme 7 in my tanks, stuff works great and their customer service helped me out a lot.

Dan    Hastings, ON

6/25/2013 6:18:59 PM

good article. I am presently experiencing a nitrite spike. My ammonia levels have finally dropped to near zero (0.25ppm) Nitrite is peaking about 5.0ppm, nitrate is on the rise, sitting about 20ppm or so.
Do I need to do a water change? Or do I just let this play its course?

Lisa    Dayton, VA

5/19/2011 2:51:00 AM

Thanks so much for the column. I'm new to this world and have kept tests records and noticed the spike in ammonia and nitrite that you mentioned. However, they both seem to be going down. (they never were at dangerous levels though). Your article really helped me to understand the cycle - I was starting to think I was doing something wrong and the fish world wasn't for me.

ethan    brownsville, TX

7/7/2010 3:32:13 PM

gotta love that nitrogen cycle. lol without it we wouldnt have aquariums

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