Nitrogen Cycle

Explaining the nitrogen cycle required an introduction to aquarium filtration.   The three types of filtration are: mechanical, chemical, and biological.  Mechanical filtration is the removal of particles from the water, while chemical filtration removes pollutants with carbon and resins.  Biological filtration is the neutralization of fish waste.  Of these three, biological filtration is the most important, because the fish will not survive in an aquarium without it.

When an aquarium has just been set up, the environment is very clean.  In fact, it is too clean to support a full load of fish.

Two main species of bacteria need to grow in the aquarium before it can be fully stocked.  These good bacteria enter the aquarium with the first fish purchased.   They come in the water from your fish store and in the waste the fish produce.

The waste which supplies these good bacteria is the reason why the bacteria are so necessary.  Fish produce ammonia in their solid waste and excrete it through their gills.  Over time, the ammonia builds up to toxic levels.  In response to this, fish produce extra mucous to protect their gills, which makes it difficult for them to breathe.  If there are too many fish in the aquarium, or the ammonia level gets too high, the fish will suffocate.

This is where the first bacteria comes in.  Nitrosomonas bacteria begin to grow and reproduce in the aquarium.  As they do, they "eat" ammonia.   After a period of time, there are enough bacteria in the tank to remove the ammonia as the fish produce it.

Unfortunately, we aren't through the cycle yet.  While Nitrosomonas bacteria remove ammonia from the aquarium, they produce nitrite.  This can also be toxic to fish.

Fortunately, another species of bacteria begins to take effect.  Nitrobacter bacteria pick up where the Nitrosomonas left off.  They eat nitrite and produce nitrate.  Nitrate is considered less toxic to fish.

The time from the introduction of the first fish until both the ammonia and nitrite have returned to zero is referred to as the cycle time.  for freshwater aquariums, this takes about 4 weeks.  The cycle time for a marine aquarium is generally 4 to 6 weeks.

After the initial cycling, you can add more fish to your aquarium.  Each time you add more fish, the aquarium has to re-cycle.  This occurs as the bacteria populations adjust to the increased levels of ammonia and nitrite produced by the new fish.

The number and species of fish used to cycle the aquarium vary between freshwater and saltwater.  For freshwater, 1" of fish for every 5 gallons works well.  We recommend mollies, platies, swordtails, danios, and some species of rasbora and tetra.   Your A World Of Fish sales representative can point out many hardy species of fish which are often used.

For marine aquariums, the ratio should be closer to 1" of fish for every 20 gallons of water.  Damsels are the best fish to use when cycling marine aquariums, although percula clowns can also be used.

And remember, the total capacity of a freshwater aquarium is approximately 1" of adult fish length per gallon.  For a marine aquarium it is 1" of adult fish length per 3 gallons.  However, these numbers will vary widely with the types of filtration which are used and the species of fish which are kept.

If you have an questions about the nitrogen cycle, or which fish will work best for your specific aquarium setup, ask any A World Of Fish sales representative.  We will assist you in selecting fish which are hardy, healthy, and compatible.  We can also direct you to aquarium test kits which allow you to track the cycling of your aquarium by measuring the levels of ammonia and nitrate.

Graph and diagram also included.