The Nitrogen Cycle

By Chelsea Gray

When starting a new tank, you must wait a month before you can fully stock it with fish. The reason for this waiting period is because you need to build up your tank’s biological filtration. The biological filtration is what decomposes dangerous ammonia into generally harmless nitrate.

Before discussing the nitrogen cycle in detail, let’s look at the three main components: ammonia, nitrite, and nitrate.

Ammonia

Ammonia, or NH₃, is the natural product of fish waste. When fish are added to a new aquarium (an aquarium without a built up biological filter), the ammonia will build up. Ammonia is very harmful to fish and should be kept at a level close to zero.

Nitrites

Nitrites (NO₂) are the byproducts of nitrifying bacteria. Like ammonia, nitrites are always very harmful to fish and should be kept undetectable.

Nitrates

Finally, nitrites are converted to nitrates (NO₃). Unlike ammonia and nitrites, nitrates are harmless in small amounts. Regular partial water changes, filter maintenance, and careful feedings will keep nitrates at acceptable levels.

Now, let’s take a look at how the Nitrogen cycle occurs in your tank.

First Stage

The first stage of the nitrogen cycle occurs when you put your first fish in the tank. As their waste products produce ammonia, the ammonia spikes. This spike leads to an increase in bacteria that consume ammonia (A bacterial bloom that makes your tank appear white and cloudy is a common occurrence during this stage). After the bacterial bloom, there is a sharp decrease in the amount of ammonia in the tank.

Second stage

As the ammonia levels decline the nitrite levels will spike. After a few days, the nitrite levels will decline sharply.

Third Stage

Finally, there will be a spike in nitrates. At this stage, you will be able to do a 25% water change. If you test your water and see that your ammonia and nitrites are at zero, and your nitrates are at acceptable levels, then your tank will be ready to fully stock with fish.

How Long Does the Nitrogen Cycle Take?

It takes about a month for your biological filtration to become established without the help of products.

Are There Ways to Speed This Up?

There are plenty of products that contain bacteria that can be added to tanks to help speed up the nitrogen cycle. For freshwater, this often involves products like Tetra’s Safe Start, a product containing water condition and live bacterial cultures. If you are keeping saltwater, then “live sand” or “live rock” (sand and rock containing nitrifying bacteria) can be added to the tank to help establish a bacteria colony.

How Do I Keep My Bacteria Colony Healthy?

Biological filtration is vital to the overall health of your fish tank. The best way to ensure that you keep your tank healthy is to do regular water changes as this keeps nitrates at acceptable levels. Also remember to never to do a water change and change the filter media on the same day. This will remove too much of your bacteria colony and will result in an ammonia spike. Never remove more then 50% at one time, as this will also remove too many nitrifying bacteria.

If you keep your bacteria colony healthy then remember not to overfeed, to perform regular water changes, and not to overstock your tank. By keeping those rules in mind, your water quality should stay great for years.