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Explaining the Nitrogen Cycle

Why is my water cloudy?

White, cloudy water can be caused by several things, but most commonly it is due to a "bacteria bloom." A "bacteria bloom" is usually associated with "new tank syndrome". Ammonia builds up in the aquarium and the nitrogen cycle begins. As the aerobic bacteria establishes itself, it floats through the water creating a cloudy appearance. A "bacteria bloom" can also be caused by sudden increases in ammonia due to overfeeding or excess organic waste and decay. Losses of large numbers of bacteria due to power outages or other circumstances can also cause blooms. 

Test the aquarium water for ammonia and nitrite. If either of these compounds are present, a bacteria culture should be added. Do not do a water change unless levels are dangerously high, or fish show signs of stress. Changing water will only lengthen the time needed for the bacteria to establish itself. 

If the tank is an established aquarium (livestock has not been added in the past 2 months or longer) be sure you are not over feeding. If the problem persists there may be too many fish in the aquarium for the biological filter to adequately handle. This forces the bacteria to float freely throughout the aquarium. Additional biological filtration will need to be added or some fish may need to be removed from the tank. 

The Nitrogen Cycle

The Nitrogen Cycle

The nitrogen cycle is the most important and fundamental principal of controlling a closed aquatic environment. No one should begin an aquarium without fully understanding what the nitrogen cycle is and how it works. The illustration and the description explain the four steps in the nitrogen cycle.

Fish waste, excess food and other decaying organic material break down into a toxic chemical compound called ammonia. Even in low levels, ammonia will increase the breathing rate of fish by irritating gill tissues. Damage to the body tissues of both fish and invertebrates will follow, causing disease and death.

Aerobic (oxygen needing) bacteria (Nitrosomonas) convert ammonia into nitrite. Nitrite is also a toxic chemical compound, equally as harmful to fish as ammonia. Nitrite destroys the hemoglobin in the blood of fish and invertebrates. Without hemoglobin the blood cannot carry oxygen.

The nitrite is converted by a second aerobic bacteria (Nitrobacter) into a far less toxic compound called Nitrate. Nitrate levels in excess of 50 ppm in freshwater or 10 - 15 ppm in marine aquariums can cause stress, encourage disease and stunt growth. Be sure to regularly monitor nitrates with a nitrate test kit.


Nitrates can be removed by several means. Small amounts are absorbed naturally by plants and algae. The remaining nitrates can be effectively eliminated with a good aquarium maintenance program. Regular water changes, cleaning filter cartridges, vacuuming substrates and removing detritus (organic waste buildups) will solve most nitrate problems. If problems persist, nitrate removing medias, denitrators and protein skimmers may be needed. Denitrators will biologically remove nitrates while protein skimmers will remove organic waste before the Nitrogen Cycle breaks them down into nitrate. (Due to the low nitrate levels required in saltwater aquariums a protein skimmer is recommended as a basic piece of equipment.) Note: Some home water supplies contain nitrates. Water changes with this water will not be effective. A reverse osmosis or deionization unit may be necessary in these cases.

Starting the Nitrogen Cycle

When starting a new aquarium, it is important to establish a good "bacteria bed" for the nitrification process. This is best accomplished through the slow introduction of fish (or live rock in reef aquariums) and the use of bacteria cultures available from various manufacturers.

A good source of oxygen and a large amount of surface area to house this bacteria must be provided. Filters like undergravel filters and wet/dry filters provide the ideal environment. Various filter medias are also available to provide housing for aerobic bacteria in canister filters, power filters and internal filters. Aquarium filters and filter medias are limited to the amount of bacteria they can house, thus limiting the amount of fish that can safely be kept in the aquarium. This makes testing the water and limiting the number of fish kept in an aquarium extremely important.

Recommended Items for a Successful Aquarium:

Freshwater Aquariums
Fish Tank
Fish Food
Aquarium Filter
Aquarium Heater
Aquarium Lighting
Test Kit
Aquarium Decorations
Water Conditioners
Maintenance Equipment

Saltwater Aquariums
This uses much of the same equipment as freshwater. Additional equipment is needed and optional equipment found under Saltwater Reef Aquariums may also be beneficial.
Additional required equipment is as follows:
Marine Salt
Sand or Substrate
Fluorescent Bulb
Protein Skimmer
Note: It is a good idea to use one size larger canister or power filter than is recommended for a freshwater aquarium of the same size.

Saltwater Reef Aquarium
or saltwater aquarium with invertebrates
The following equipment in addition to those listed under Saltwater Fish Aquariums is recommended:
Wet/Dry Filter or Trickle Filter
Protein Skimmer
Metal Halide Lighting or High Quality Fluorescent Lighting

Additional Equipment:
Test Kit
Salt Mix
Aquarium Heater
pH Buffer
dKH Buffer
Bacteria SupplementMaintenance Equipment
Fish Food
Aquarium Decorations
Gravel or Substrate is not recommended in reef ­aquariums with live rock. Live sand can be used in certain systems.