Nitrogen Cycle for Freshwater

Once an aquarium has been running for twenty-four hours, there is only one step left before introducing livestock. You need to perform a water test.

Why test water?

An aquarium is a closed system and, unlike a stream where water is constantly removed and replenished, an aquarium will build up waste and organic compounds over time.  Testing the aquarium water helps you know what is happening in the aquarium.

What needs to be tested?

The basic tests to perform regularly on aquarium water are pH, Ammonia, Nitrite and Nitrate.

A simple understanding of pH is helpful to keeping fish healthy. The term pH refers to how acidic or basic a substance is. The pH scale ranges from 0 to 14; 7 is neutral. A pH reading lower than 7 indicates acid, a reading higher than 7 indicates base.

  Nitrogen  Cycle , Aquatic Experts

Fish require certain pH levels depending on where they originally lived in the world. For example, tetras prefer a slightly acid environment, while guppies prefer a neutral pH.  Discuses prefer a pH of 6.0 while many African Cichlids want a higher pH of 7.8 or more.  This is one good reason why Discus cannot be housed with African Cichlids in the same aquarium.

Most tropical fish prefer gradual changes in pH but you can adjust an unpopulated aquarium very quickly. Should a change in pH be necessary, a buffering agent can be added to the water. Once the pH is at the level for the customers chosen fish, they are ready to add them

Most community fish prefer pH to remain between 6.7 and 7.0.  Test the water before adding fish. If necessary, adjust pH before adding fish.

Other test kits are used after the fish have been added to the aquarium and are helpful in determining how a tank is cycling. These test kits measure compounds that are harmful to the fish. 

Note: For the first few weeks it is set up, a new aquarium is a stressful environment to fish. Knowing this can be a key to successfully selecting the first fish.


The Cycle

A basic understanding of the aquarium cycling process will help you save the lives of many fish. Once the fish are added to an aquarium, it will begin a process called the Nitrogen cycle, or cycle for short. The cycle will not begin until there are waste products (from the fish and fish food) present in the water. While harmful to fish, these compounds provide food to a beneficial class of bacteria.

The process is relatively simple.  After fish are added to an aquarium, fish waste (ammonia) begins to build up. As the ammonia level increases, it becomes more toxic to your fish.  Nature provides a natural ammonia reducer, bacteria called Nitrosomonas. These beneficial bacteria break down ammonia into nitrite. Nitrite is also harmful to fish, but other bacteria, called Nitrospira, use nitrite and give off a byproduct called nitrate. Nitrate is less toxic to fish than either ammonia or nitrite and can be utilized by plant life or removed with simple water changes.

It takes from 4 to 6 weeks for beneficial bacteria to become established when aquarium water is at a temperature of 80º. This process may take longer if the temperature is lower.  Because high levels of ammonia and nitrite are stressful to fish, no new fish should be added once the cycle begins and it normally is not necessary to remove water and add newly conditioned water during this time (except in cases of extremely high levels)  After the cycle finishes, (when both ammonia and nitrite levels are zero) a water change is recommended and more fish can then be added.

Several bacteria starters are on the market today which can shorten cycling time and keep the ammonia and nitrite levels from rising as high.These products contain live cultures of beneficial bacteria and are added directly to the aquarium water. 

This chart illustrates the cycle process.

The actual levels of ammonia, nitrite and nitrate may be higher than shown. There are many factors that determine how high each of these levels will go such as the number and size of fish, how often and how much they are fed and how fast the beneficial bacteria become established.

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