The Nitrogen Cycle

Nitrogen Cycle in the Freshwater Aquarium

The flow of Nitrogen (N) in an aquarium is a critical component to the health and well-being of your aquatic pets. A basic understanding of this process is a necessary part of starting a successful aquarium. There are three steps that make up the Nitrogen Cycle as it functions in the freshwater aquarium:

Step One – Fish Produce Waste

Fish and invertebrates produce waste as a result of normal biological processes. In regard to Nitrogen, this waste is excreted in the form of Ammonium (NH4+). In hard water with a high pH (above 7.0) Ammonium is converted to toxic Ammonia (NH3). Ammonia is highly toxic to fish and invertebrates. Any measurable amount in an aquarium with live animals is a danger to their health, and exposure can result in “Ammonia Burn” or even death. Good thing there are beneficial bacteria that “cleanse” this harmful chemical from the water! Aquatic plants in the aquarium also “cleanse” the water by absorbing Ammonium. The availability of Ammonium for use by plants can be reduced in hard water with high pH.

Step Two – Nitrosomonas Convert Ammonia to Nitrite

Beneficial bacteria known as Nitrosomonas live on the surfaces found in the aquarium such as in the substrate, on the leaves of plants, the glass walls, and especially in the filter media. This group of bacteria use Ammonia as a means of producing energy in the presence of Oxygen (O2) in the water. As a result of this process, Ammonia is converted into a somewhat less toxic chemical called Nitrite (NO2). Nitrite is not as harmful as Ammonia, but measurable levels of this chemical in the aquarium also cause harm to fish and invertebrates. Again we are fortunate that another group of beneficial bacteria convert harmful Nitrite into a third, relatively harmless chemical.

Step Three – Nitrobacter Convert Nitrite to Nitrate

Nitrobacter, another form of beneficial bacteria, also lives on the surfaces within the aquarium. Nitrobacter derive energy from converting Nitrite into Nitrate (NO3) in the presence of Oxygen. Nitrate is relatively harmless to fish and invertebrates, and in small concentrations does not seem to cause any harm. Please Note: fish and invertebrates that have grown accustomed to very high levels of Nitrate can be “shocked” when transferred to water with very low or no measurable Nitrates. This can cause unhealthy stress to the animal. Excess Nitrates can be removed from the aquarium through regular water changes and/or by consumption by aquatic plants. Aquaponics farms use Nitrates produced by this process to fertilize crops. Neat, huh?

Bonus Step – Anaerobic Bacteria Convert Nitrates into Nitrogen Gas

A third way Nitrates can be removed from an aquarium is through anaerobic bacterial processes. In anaerobic (lacking Oxygen) conditions within the substrate or filter media, anaerobic bacteria can break apart Nitrates into Nitrogen Gas (N2) in order to obtain the Oxygen they need to live. This Nitrogen gas eventually rises to the water surface and is released into the atmosphere.

1. Fish food is eaten by fish, 2. fish produce ammonia, 3. nitrosomonas turn ammonia into nitrite, 4. nitrobacter turn nitrite into nitrate, 5. nitrate is reduced by plants, anaerobic bacteria, and water changes.

What does this mean for my aquarium?

One of the first, most critical steps in setting up a successful aquarium is allowing the tank to properly “cycle” before adding fish or invertebrates. When someone says a tank is “cycled,” what they mean is that the aquarium has gone through the process of seeding the two kinds of beneficial bacteria necessary to efficiently convert harmful Ammonia to harmless Nitrates. The time required to fully cycle a tank is different for every tank (from a matter of days to a matter of weeks). The time required for a tank to cycle is influenced by a number of factors including size the of the tank, amount of available nitrogen (in the form of Ammonia) and concentration of bacteria at the beginning of the process.

One way to dramatically reduce the time necessary to fully cycle an aquarium is by transferring used substrate or filter media from a fully established tank to the new tank. Simply transferring used aquarium water to the new tank will not suffice, as the beneficial bacteria live attached to tank surfaces, not free-floating in the water.

Transferring plants and other decorations from an established tank will also help to quicken the process. Another way to speed up the process is by raising the temperature of your tank water. The ideal temperature for bacterial growth is 86 – 95 F (30 – 35 C), but this is not an ideal range for most fish. So this technique should only be used in a fishless cycle scenario (explained below).

There are a few different techniques that aquarium hobbyists use to cycle a new tank. The three most common ways are:

  • Fishless Ammonia Cycling – This advanced technique involves adding small amounts of 100% pure Ammonia to the tank water while monitoring the levels of Ammonia, Nitrites, and Nitrates. You can purchase a bottle of pure ammonia from most grocery or hardware stores, but you must make sure there are no additives like scents or soaps. Chances are the ammonia you find in stores will be watered down to reduce its potency. This is OK, you may just need to add more each day.
    • Begin by steadily adding small amounts of Ammonia to the water. Start with just a few drops each day, monitoring the levels of ammonia. Try to maintain a level of about 2ppm.
    • Keep adding Ammonia each day to maintain 2ppm until the level begins to decline (check before adding the new ammonia). You’ve got nitrosomonas! Now you can begin measuring for nitrites. Continue to add a couple drops until the level of Nitrites also begins to drop and Nitrates can be measured.
    • Once you are able to measure Nitrates, you’ve got nitrobacter! Stop adding Ammonia and monitor the three levels. When Ammonia and Nitrites are near 0 ppm but Nitrates are still measurable it is safe to begin slowly adding fish and invertebrates.
    • You should not add many animals all at once, because while your tank now has the necessary bacteria colonizing your filter media, you may not have a large enough colony to support a full load of fish. This colony will grow as time goes on, until a balance is formed.
    • You will need to do regular water testing and water changes once fish have been introduced to keep levels of all three chemicals within healthy parameters. DO NOT ADD HOUSEHOLD AMMONIA TO AN AQUARIUM CONTAINING LIVE FISH OR INVERTEBRATES! THIS COULD VERY EASILY KILL THEM.
  • Fishless Food Cycling – This technique involves adding small amounts of fish food to the tank water while monitoring levels of Ammonia, Nitrites, and Nitrates. This method is similar to the Fishless Ammonia Cycling method, however it is slower. Uneaten food in the aquarium will begin to break down and release Ammonia into the water, starting the cycling process. Do not add too much food or the water will become foul with rot. It doesn’t really matter what kind of food you use for this. Flake or pellet food work well. For larger tanks, I’ve heard of people throwing a cocktail shrimp into the tank! I can’t say I’ve tried this, but I can imagine the smell is…. interesting.
    • First add a small pinch of flake or pellet food to the water. During the following hours and days, it will begin to break down and not look very pleasant. Wait a day or two and then measure your ammonia level.
    • Keep checking your ammonia level every day or so until it stops increasing. If the level is below 2ppm, add a little more food and continue checking periodically.
    • If you notice your ammonia level beginning to go down, you’ve got Nitrosomonas! Now you can begin measuring your nitrite levels.
    • You may need to add a little more food until the nitrobacter take root and you begin seeing a decrease in your level of nitrite.
    • When your nitrites begin to reduce, check for nitrates. If you are able to measure nitrates, clean the remaining food out of your tank and begin adding fish.
    • Please Note: depending on how long the process takes and how much food you are introducing to the water, you may need to clean some of it out as the process goes on, so the water doesn’t become too foul. It is a good idea, once the tank has all the necessary bacteria, to do a large 50% + water change to get a “fresh start” before adding fish. Don’t worry, you won’t lose your bacteria, because they live on the aquarium surfaces, not suspended in the water.
  • Fish-in-Tank Cycling – This technique involves putting fish into the tank right from the start and cycling with fish waste as the Ammonia source. Most hobbyists use this method simply because they don’t know any other way. This method is somewhat controversial because some feel it is cruel to cycle a tank using a living creature. That is for you to decide. If you choose to use this method, you will need to carefully monitor your aquarium’s Ammonia, Nitrite, and Nitrate levels and perform frequent water changes to keep the water relatively “safe” for the fish.If your ammonia level goes above 1ppm, you should do a partial water change, maybe 20%. This may make the process take longer, but it will put less stress on the fish. It is recommend if you choose this method to only use one or two fish, perhaps “feeder fish” or snails. Once the tank is cycled you can begin to add more fish.

I personally prefer to add a used sponge filter, substrate, decorations, and other material from an established tank to kick-start the process. These objects (especially the sponge filter) will come with loads of bacteria already growing on them, provided they have not been allowed to dry in air. I have had much success doing this and putting a small number of live creatures into a relatively new tank (within a few days). Many pet stores will sell you a “seeded” sponge filter to help get you started with a new tank. Can’t hurt to ask!

Plants not only absorb ammonium and nitrates, they also come with their own colony of bacteria. Like any other surface in an established aquarium, plants will have bacteria growing on their leaves and stems. This is one the ways plants are powerful allies in maintaining a healthy aquarium.

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