How to Maintain an Aquarium's Nitrogen Cycle

Updated on May 22, 2019
Jana Louise Smit profile image

Jana worked in animal welfare with abused and unwanted pets. She loves sharing her hands-on experience regarding domestic and wild critters.


Nitrogen Cycling in a Nutshell

Dealing with a nitrogen cycle is necessary for keeping an aquarium healthy. In order to do the best job, one must have a basic understanding of the following:

  • Where the nitrogen comes from
  • The chemistry of the cycle (we'll keep it short and sweet)
  • The role of beneficial bacteria
  • How to condition the tank with bacteria (very easy)
  • How to speed up conditioning
  • Why pH testing is a chore that's here to stay
  • How to correct unbalanced pH

Patience Is a Must

Many enthusiasts get bitten by the aquarium-bug when they stand in a pet store. Often this leads to impulse buys and dead fish. The latter results from something called “new tank syndrome.” This is basically incorrect water conditions that shock fish to death. To avoid this, one must knuckle the nitrogen cycle into a safe zone for fish and the bad news is that the process can take weeks.

Where the Nitrogen Originates

Not to be foul, but it starts with fish poop. For this reason, it is better to start small with “starter fish.” The intensity of the cycle is directly tied to the amount of waste expelled every day. Setting up a tank with a few hardy fish, like two or three guppies, will give them and the new owner a chance to condition the tank. Once settled, more fish can be added and monitored.

The Chemistry of the Cycle

  1. The fish answers nature's call
  2. The waste builds up as ammonia and carbon dioxide
  3. These chemicals are deadly in high amounts
  4. To remove them the tank must be conditioned with bacteria
  5. Beneficial bacteria break the ammonia down to less harmful nitrates
  6. The CO2 can be removed by plants, airstones, and filters
  7. Tests determine the water quality (pH)
  8. Corrective steps are taken if tests deem so necessary

Starter Stars

Guppies make excellent starter fish because they are remarkably tough and adaptable.
Guppies make excellent starter fish because they are remarkably tough and adaptable. | Source

The Role of Bacteria

Two kinds of bacteria condition the tank. When ammonia builds up, a species called “Nitrosomonas” breaks down ammonia into nitrites, which is a little less toxic. At a later stage, another bacteria called “Nitrobacter” convert the nitrites into an even less harmful substance; nitrates. Although nitrates can also be deadly in high concentrations. Deadly concentrations of nitrites and nitrates can be controlled through regular water changes.

Establishing a Bacterial Colony

Luckily, one needn't go out and buy these beneficial boogers. They are already in the water. However, in new aquariums, their numbers are too low to efficiently deal with waste. For this reason, you must take care of your growing bacterial colony and it's quite easy.

It takes about two weeks for your starter fish to kick off the ammonia levels necessary to interest the bacteria. They multiply rapidly to break down the dangerous chemicals. The first-phase bacteria, the Nitrosomonas, need plenty of oxygen to thrive. Make sure the aquarium is equipped with the necessary filtration and airstones to provide the oxygen the organisms need for healthy growth. This is all the second-phase bacteria, Nitrobacter, requires as well.

Speeding Things Up

This is completely optional. Some people just like to get their aquariums ready faster and there are several options available to them. Even so, conditioning times vary, depending on the size of the aquarium, water temperature and the number of starter fish. In general, it takes between four to six weeks.

  • Extra food. When uneaten fish food decays, it also adds to the ammonia of the tank. Be careful not to overdo it as a growing bacterial colony can only deal with so much ammonia
  • Mature gravel. This gravel comes from a tank where the nitrogen cycle and bacteria are well established (the donor aquarium must be disease-free)
  • There are commercial products available for this purpose, but generally, they don't speed up things that much
  • Gradually add a fish or two

Whether you go the slow or the fast route, at one point the water might develop a cloudy look. This is a healthy bloom of bacteria and the aquarium's filters should clear the water again within days.

Testing Water Quality

To keep fish healthy, the water must be tested on a regular basis. Available at pet stores or online, such kits test for pH, ammonia, nitrites, and nitrates. The first test should be done as soon as the tank is set up. During the conditioning phase, daily tests track the progress of the nitrogen cycle (note that it is normal for pH to drop at first). Once conditioning is complete, weekly tests are necessary for as long as the aquarium exists.

Depending on one's budget, pH test kits range from simple strips to electronic probes that give digital readings.
Depending on one's budget, pH test kits range from simple strips to electronic probes that give digital readings. | Source

Understanding pH

The pH scale shows how acidic or alkaline water is. At the bottom, 0 is considered highly acidic and 14 is highly alkaline. Neutral is marked by 7. The number one should aim for depends on the species of fish destined to occupy the aquarium once the tank has cycled.

The important thing to remember is that freshwater pH can rapidly shift and even a single point (say, a drop from 6 to 5) can be deadly. Once you know the pH values for your fish, aim to establish this number and do regular tests to keep it steady.

Correcting Unbalanced pH

The job of removing ammonia cannot entirely fall to bacteria. Frequent water changes are best to keep toxic chemicals under control. Aim for a five percent daily or fifteen percent weekly change.

When numbers show that your pH is too low, or acidic, try the following.

  • Some pH kits come with sodium bicarbonate, or baking soda, for this purpose. Follow the directions on how much to add
  • Do a water change

When numbers show that your pH is too high, or alkaline, try the following.

  • Some kits have sodium biphosphate for this purpose. Follow the instructions on how to use it safely
  • Add de-mineralized water

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2019 Jana Louise Smit


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