The Nitrogen Cycle

When fish food decomposes or when fish excrete waste, nitrogen compounds are released into your tank’s water. The compounds are broken down into other compounds by bacteria. The final product, NO₃, is either absorbed by plants or removed through gravel siphoning. This process is called nitrification.

A new tank will not have any nitrifying bacteria present (unless gravel or filters from a previously established tank are used). You can use a biological filter to bring in nitrifying bacteria. Once a few fish are added to the tank, the nitrogen  begins. Ammonia and ammonium start building up and can rise to a toxic level. Nitrosomonas bacteria from the filter begin to grow. These bacteria break down the ammonia into nitrite (NO₂), which is less toxic. Nitrobacters also develop from the filter, and they break down the nitrites (NO₂) into the less toxic nitrate (NO₃). Nitrates are absorbed by plants or algae or are removed by a water change. This process is diagrammed below.

Nitrogen Cycle

QUESTIONS

• How are nitrogen molecules produced in your aquarium?
• What other substances do bacteria break down ammonia to?
• Why are biological filters important?
• Why is it important that you change your tank’s water?