The "water cycle" in an aquarium, more accurately known as the aquarium nitrogen cycle, is a crucial biological process that makes the water safe for fish and other inhabitants.
This cycle is a biological process that begins when a source of ammonia (such as fish waste or food) enters the aquarium. It involves the natural transformation of harmful compounds into less harmful ones through the action of beneficial bacteria.
Understanding the Aquarium Nitrogen Cycle
The cycle progresses through distinct stages, driven by different types of bacteria that colonize surfaces within the aquarium, such as filters, substrate, and decorations.
The Stages of the Cycle
The process converts toxic waste products into safer substances:
- Ammonia Production: The cycle starts with the introduction of ammonia (NH₃), a highly toxic compound. Sources include:
- Fish waste
- Uneaten food
- Decaying plant matter
- Ammonia to Nitrite Conversion: Beneficial bacteria, specifically Nitrosomonas species, consume ammonia and convert it into nitrite (NO₂⁻). Nitrite is also harmful to fish, although generally less so than ammonia in lower concentrations.
Thanks to beneficial bacteria and plants, ammonia is converted to nitrite. - Reference
- Nitrite to Nitrate Conversion: Another type of beneficial bacteria, primarily Nitrobacter species, consumes nitrite and converts it into nitrate (NO₃⁻). Nitrate is significantly less toxic to fish at typical aquarium levels compared to ammonia and nitrite.
Nitrite is then converted to nitrate. - Reference
Once nitrate is produced, the initial cycle is complete.
Once nitrate is produced, the cycle is complete. - Reference
The Role of Bacteria and Plants
- Beneficial Bacteria: These microscopic organisms are the workhorses of the nitrogen cycle. They establish colonies and perform the chemical conversions of ammonia to nitrite and nitrite to nitrate. Establishing a healthy population of these bacteria is known as "cycling" an aquarium.
- Plants: As mentioned in the reference, live aquarium plants also play a role. They can absorb nitrates from the water as a nutrient source, further helping to keep nitrate levels down.
Visualizing the Cycle
Here is a simplified view of the transformation:
| Stage | Produced From | Converted To | Toxicity (to fish) |
|---|---|---|---|
| Ammonia (NH₃) | Fish Waste, Food | Nitrite | High |
| Nitrite (NO₂⁻) | Ammonia | Nitrate | Moderate |
| Nitrate (NO₃⁻) | Nitrite | Removed by plants/water changes | Low (at typical levels) |
Why is this Cycle Important?
Understanding and establishing the nitrogen cycle is fundamental to successful fishkeeping. Without it, ammonia and nitrite would quickly build up in the water, poisoning fish. A "cycled" tank has enough beneficial bacteria to process the waste produced by the fish, maintaining a healthy and stable environment. Regular partial water changes are still necessary to remove accumulated nitrates and replenish essential minerals.
