The Nitrogen Cycle in Aquariums

In natural ecosystems inhabited by common tropical fish, toxic levels of nitrogen-containing compounds are rare. However, in an aquarium, overfeeding and overcrowding are common. These confined spaces often lead to nitrogen pollution, which can harm or kill your fish. Here are the basic compounds and processes that comprise the nitrogen cycle.

Nitrogen Cycle

The natural nitrogen cycle involves nitrogen moving from air to plant, to animal, to bacteria, and back to air, requiring no human intervention. In an aquarium, this process is more a biochemical cascade, where nitrogenous compounds are continually broken down from ammonia to nitrite to nitrate. The final nitrates are absorbed by aquarium plants or removed from the water by other means.

This cascade illustrates how natural wastes are processed in ecosystems. In a closed aquarium, it's essential for the hobbyist to establish and maintain this cascade. Ammonia, nitrite, and nitrate are the primary biological toxins in an aquarium, so the nitrogen cycle must effectively convert and remove these byproducts.

In a living aquarium, this cascade establishes over time, typically taking up to three months for a new setup to convert wastes into nitrate fully. Stocking the aquarium gradually with young, small fish allows nitrogen-converting bacteria to grow and keep pace with increasing waste.

Ammonia

Fish urea and proteins are swiftly converted by bacteria (step 1) into ammonia, a colorless, pungent, and highly toxic gas. Ammonia levels rise if there are too many fish or if they are overfed. In a balanced aquarium, 'nitrogen-fixing bacteria' oxidize this ammonia (step 2), converting it to nitrite.

Nitrite

Nitrites are common killers of aquarium fish and are therefore crucial to manage in the nitrogen cycle. They form through the partial oxidation of ammonium ions. Nitrite-loving bacteria then convert nitrite into nitrate (step 3), making it mostly harmless.

To prevent nitrite build-up, feed sparingly and ensure the tank isn't overpopulated. Regularly perform a partial water change (not exceeding 20% of the total volume) using well-aged water, not tap water.

Ensure there aren't too many animals in the aquarium. Beginners often overlook that even 'cleaner fish' like catfish, algae eaters, and snails produce waste, contributing to total nitrite levels.

Nitrate

Nitrates are the end-products of nitrogen compound oxidation. In aquariums, they mainly arise from the breakdown of animal protein and ammonium compounds, such as urine, excrement, food scraps, and dead fish, snails, and plant leaves.

Most freshwater tropical fish and other aquarium inhabitants tolerate large quantities of nitrates. However, to prevent excessive nitrate build-up, feed sparingly and maintain a small animal population.

Plants

Aquatic plants actively use nitrogen and can significantly reduce nitrate levels in a well-adjusted aquarium. In natural ecosystems, plants remove and use nitrates. In an unplanted tank, the owner must manage nitrate removal at this final cascade stage.