Understanding the nitrogen cycle in aquariums: From ammonia to nitrate

If you’re into aquariums, you’ve likely heard that you must cycle the aquarium while setting it up. But what does it mean? The phrase refers to establishing the nitrogen cycle, an essential biological process that keeps your aquarium’s water quality in check by converting harmful waste products into less toxic forms.

In this blog post, we’ll explain the nitrogen cycle in detail and discuss how you can manage it to maintain a healthy aquarium environment.

What is the nitrogen cycle?

The nitrogen cycle is a natural process that occurs in every environment where there is nitrogen. In a broader biogeochemical sense, we care about your fish tank. In this context, the nitrogen cycle refers to converting fish waste products into substances that plants can consume.
The nitrogen cycle involves a series of chemical reactions performed by beneficial bacteria. These bacteria convert harmful ammonia (NH₃) produced by fish waste, uneaten food, and decaying plant matter into less toxic nitrite (NO₂^–), and ultimately into nitrate (NO₃^–).
The nitrogen cycle consists of three main stages:

1. Ammonia Production: Fish waste, uneaten food, and decaying organic matter in your aquarium break down, releasing ammonia. Ammonia is highly toxic to fish, even in small amounts, and can cause stress, illness, or death. You wouldn’t want to swim in your own waste, right?
2. Nitrite Formation: Beneficial bacteria called Nitrosomonas convert ammonia into nitrite. Nitrite is also toxic to fish, although less so than ammonia, and can cause similar health issues.
3. Nitrate Formation: Another group of beneficial bacteria called Nitrospira convert nitrite into nitrate. Nitrate is relatively non-toxic to fish in low concentrations. However, high levels can lead to poor water quality and health issues for fish and plants. That’s why you still need to do partial water changes.

How to establish the nitrogen cycle in your aquarium

Before adding fish to a new aquarium, you must establish a healthy nitrogen cycle, a process known as “cycling.” This involves introducing beneficial bacteria to the tank and allowing them to colonize the aquarium’s filter, substrate, and other surfaces.
There are several methods to cycle an aquarium:

1. Fishless Cycling: This method involves adding ammonia or a source of ammonia (e.g., fish food) to the aquarium without any fish. This process provides food for the beneficial bacteria, allowing them to multiply and establish the nitrogen cycle. It typically takes 2-3 weeks for the cycle to establish.
2. Fish-in Cycling: This method involves adding a few hardy fish to the aquarium to produce ammonia naturally. It requires diligent monitoring of ammonia and nitrite levels and frequent water changes to prevent harmful concentrations. Fish-in cycling can be stressful for the fish and is generally not recommended for beginners. It probably exists only because people who set up their first aquarium usually can’t wait long enough for fishless cycling to complete.

That being said, you don’t have to do anything in particular. Set up a fish tank, and, before adding fish, add some fish food, and wait.

Maintaining the nitrogen cycle

In an ideal world, once established, a nitrogen cycle would indeed be a stable cycle: fish produce waste, bacteria transform it into plant food, plants grow, fish eat plants, and the cycle continues.
However, in a typical fish tank, we constantly add more nitrogen through fish food, causing the amount of ammonia, nitrates, and nitrites to gradually rise, and something must be done about it.

1. Perform regular partial water changes: Weekly water changes (typically 10-20% of the tank volume) help dilute nitrate levels and maintain water quality. Learn more about partial water changes here.
2. Monitor Water Parameters: Regularly test your aquarium water for ammonia, nitrite, and nitrate levels. This will help you detect any imbalances and address them promptly.
3. Avoid Overfeeding: Excess food contributes to ammonia production. Feed your fish only what they can consume in a few minutes, and remove any uneaten food.
4. Clean the filter properly: Clean your aquarium filter regularly, but avoid rinsing the filter media with tap water, as this can kill the beneficial bacteria. Instead, use water from the aquarium itself.
5. Avoid overstocking: Overstocking your aquarium with too many fish can produce excess waste and ammonia, overwhelming the nitrogen cycle. Follow the general guideline of one inch of fish per gallon [note to editor: add metric in parentheses] of water for a balanced aquarium.
6. Introduce Live Plants: Live plants can help absorb nitrates, reducing their levels in the aquarium. They also provide additional surfaces for beneficial bacteria to colonize. Read more about the benefits of live plants here.

The nitrogen cycle is an essential aspect of maintaining a healthy aquarium environment. By understanding this biological process and establishing a stable cycle in your aquarium, you can ensure the health and well-being of your fish and aquatic plants.
Regular maintenance, monitoring of water parameters, and avoiding common pitfalls like overfeeding and overstocking will help you maintain a healthy fish tank.