Nitrogen Cycle for an Aquarium

A basic principle to understand for angelfish and any fish keeping is the nitrogen cycle for an aquarium. The nitrogen cycle involves fish waste and how it relates to water quality, food, nitrates, nitrites, and ammonia. Controlling this fish tank cycle is what makes any fish tank work.

The Fish Tank Cycle




Nitrogen Cycle for an Aquarium Basics

It takes a little time for the nitrogen cycle to get started in a new aquarium. Waste from the fish, and uneaten food produce ammonia to start the cycle. Ammonia and nitrites are highly toxic to fish. Nitrates, the final phase of the nitrogen cycle, are toxic at high levels.


Bacteria drive the nitrogen cycle for an aquarium and are both helpful and damaging to an aquarium habitat. Any surface area including gravel, filters, and rocks in the aquarium is a place for bacteria to grow and thrive. They break the ammonia down into nitrites (NO2) and then into nitrates (NO3) which are not as lethal to fish as ammonia. Undergravel filters work on the theory that the filter pulls water through the gravel, causing bacteria to flourish and decompose the ammonia and nitrites.

Fish Food: The Beginning of the Fish Tank Cycle

Food and other organic matter start the nitrogen cycle. It is important not to overfeed, as any waste food will convert to ammonia.

Ammonia (NH3)

Waste from fish and uneaten food turn into ammonia when broken down by bacteria and fungi. Ammonia is toxic and kills fish.

Nitrites (NO2)in Aquariums

Ammonia converts to nitrite by bacteria. Low levels of nitrite are toxic to fish. Regular water changes remove some of the nitrite and keep it at non-lethal levels.

Nitrates (NO3) in Aquariums

Bacteria further converts nitrites to nitrates. Fish tank nitrates are not typically harmful to fish unless levels build up too high. Fish can tolerate low levels of nitrate. It is beneficial to plants and helps them flourish.

Plants and Algae

Plants and algae use nitrates and some nitrites as a food in their life cycle. A heavily planted tank will help remove nitrates and nitrites from the aquarium.

Water Changes

Water changes are one of the most important maintenance procedures for a healthy fish tank. Freshwater angelfish fish require a weekly or semi-monthly change of 25 percent of the water removes nitrates and other toxic chemicals. This is a completely necessary part of aquarium maintenance.


Aquarium test kits are available to test the different parts of the nitrogen cycle for an aquarium, including for ammonia, nitrites and nitrates. It is advisable to keep one on hand to check the water periodically. This will allow you to avoid disaster by doing a water change. These kits will test for pH, nitrate, nitrite and ammonia.

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