Understanding the Nitrogen Cycle in Aquariums - The Nitrogen Cycle in an Aquarium

You are new to fishkeeping and want to get off to a good start. If the nitrogen cycle means nothing to you, take a few minutes to read this short guide to understand it better. It is the foundation for starting up or balancing an aquarium. The same applies to a garden pond.

The three stages of the nitrogen cycle

It all starts with waste. Fish droppings, leftover food and dead plant leaves break down in the water and produce ammonia, or ammonium depending on the pH. This compound is toxic to fish, and all the more so the higher the pH.

A first group of bacteria, living mainly in the filter and the substrate, converts this ammonia into nitrites (NO2). Nitrites are still toxic, as they interfere with the fish's breathing. A second group of bacteria then converts them into nitrates (NO3), which are far better tolerated.

In a planted aquarium, the plants also take up some of the ammonium directly, which takes pressure off the filter. That does not remove the need to test the water, though.

In most aquariums, the cycle stops there. Nitrates build up gradually: the plants use some of them, and water changes remove the rest. That is why an aquarium, even a well-filtered one, always needs maintenance.

These bacteria need oxygen and a surface to settle on. They do not live mainly in the water, but in the filter media. That is why a filter that has been switched off for a long time or rinsed under the tap can trigger a nitrite problem again. They also multiply slowly: no product can force them to go much faster than their natural pace, which depends on the water. A bacteria starter helps them get going, but does not replace checking the water.

Starting an aquarium, week by week

Allow about 4 weeks to balance a new aquarium. This is only a guideline, as it depends on temperature, volume and fish stocking. Here is a cautious timeline:

• Day 1: put in the substrate, decor and plants, fill the aquarium, add a water conditioner and switch the filter on, leaving it running non-stop.
• First week: add bacteria and let it run without fish. A small source of organic matter, such as a pinch of food, feeds the start-up.
• Second and third weeks: test for nitrites twice a week. They rise, then fall again: a sign that the cycle is establishing itself.
• Fourth week: once nitrites remain undetectable over several tests, introduce just a few fish.
• The following weeks: add the remaining fish in small groups, testing each time.

The tests that show where you stand

You cannot see the cycle: perfectly clear water can contain nitrites. Only a test will tell you where you stand. Always test at the same time of day, on a sample taken from the middle of the aquarium, and write down each result to follow the curve.

• Nitrites: this is the start-up test. They must stay undetectable before you add any fish.
• Nitrates: they appear once the cycle is established. After that, they help you fine-tune your water changes.
• pH and KH: these tell you how stable the water is, and too low a KH can slow the bacteria down. Very soft water deserves closer monitoring during start-up.

And what if there are already fish in the aquarium during start-up? Stop feeding for a few days, do a partial water change as soon as nitrites are measurable, keep the filter running and test every day until the reading is back to undetectable. Do not add any new fish during this period.

If nitrites come back in an established aquarium, see Nitrites coming back in the aquarium: finding the cause. To choose your equipment, also read Which water tests for a freshwater aquarium?

Maintaining the cycle once it is established

A balanced aquarium remains a living system that depends on what you do. In freshwater, changing 20 to 25 % of the water each week is generally suitable, adjusted according to your nitrate readings. Add a conditioner to tap water: chlorine can harm the beneficial bacteria.

Clean the filter media in water taken out of the aquarium, never under the tap, and avoid replacing everything at once. Feed sparingly: whatever the fish do not eat within a few minutes ends up as ammonia. After a medication treatment, check nitrites again, as some products can disrupt the bacteria. If you also look after a garden pond, the principle is the same, on a larger scale.

Three mistakes come up often: changing all the water at once, which upsets the balance; replacing all the filter sponges on the same day, which removes a large share of the bacteria; and adding lots of fish as soon as the water looks good. If the aquarium has been without power for several hours, also keep an eye on nitrites over the following days.

Frequently asked questions

How long does it take to establish the nitrogen cycle?

As a rule, allow about 4 weeks. Only your nitrite tests can confirm that the aquarium is ready.

Can I add fish on the very first day?

It is not recommended. Without established bacteria, ammonia and nitrites can rise and make the fish ill.

Should I change the water during start-up?

Without fish, it is generally not necessary. With fish and measurable nitrites, a partial water change protects them.