The Easy Guide to the Nitrogen Cycle for Aquariums

Are you getting started with your first fish tank? You may have also heard of the “aquarium Nitro cycle,” which is a series of complex scientific terms and graphs that can seem overwhelming. Don’t panic! Continue reading to learn more about the nitrogen cycle.

What is the Nitrogen Cycle in Aquariums?

The nitrogen cycle basically describes how nature creates food (in the form of microorganisms and plants), fish eat the food and produce waste, and then nature breaks down the fish waste so that it can get converted into food again.

An illustration of the nitrogen cycle for aquariums.

When aquarium hobbyists talk about the nitrogen cycle, they are usually referring to the specific part of the cycle where the fish waste turns into toxic nitrogen compounds like ammonia, nitrites, and nitrates. These nitrogen compounds can potentially kill our fish unless we make sure we have plenty of microorganisms (also known as beneficial bacteria) and plants to consume the waste products.

Let’s make an illustration using yellow, brown, or blue M&Ms as the three toxic nitrogen compounds.

– Yellow = ammonia (which is very toxic and can burn fish gills and skin) – Brown = nitrite (which is somewhat toxic) – Blue = nitrate (which is not as toxic as ammonia and nitrite)

Step 1: Whenever your fish goes to the bathroom, some ammonia is produced.

Step 2: Beneficial bacteria #1 eats the ammonia and produces nitrites.

Step 3 – Beneficial bacteria #2 eats the Nitrites and makes Nitrates (the least toxic form of nitrogen).

Step 4: The fish continue to eat food and produce waste, which gets processed from ammonia and nitrites into more nitrates.

Step 5: Eventually, the amount of nitrates will build up and can become harmful to the fish in high amounts. You must remove the nitrates either by doing a water change or by using aquarium plants. (The aquarium plants consume the nitrates to produce new leaves.)

Cycling an aquarium simply means that you need to have enough biological filter (e.g. beneficial bacteria or aquarium plants) to get rid of all ammonia and other nitrites. If you have ammonia test strips and multi-test strips, ideally you should measure 0 ppm ammonia, 0 ppm nitrites, and usually some amount of nitrates in your tank water. You should remove any tank water that has a concentration of 40 ppm or higher and replace it with clean, fresh water.

How Long Does It Take for an Aquarium to Cycle?

It depends, but usually it can take anywhere from a few weeks to months. It is possible to speed up the process by purchasing live nitrifying bacteria or acquiring used filter media from friends. You can also grow live plants which contain beneficial bacteria. Read the entire article to learn how to cycle an aquarium.

If you ask your average hobbyist whether or not their aquarium is cycled, most people think the answer is either a hard yes or no. The truth is that the answer to this question is more complicated than it seems. Instead, we should be asking, “How much beneficial bacteria does the tank have, and is it enough to treat the waste produced by the fish?” For example, if you have a “cycled” aquarium with 3 neon tetras and then suddenly you add 200 neon tetras, that aquarium no longer has enough beneficial bacteria to immediately convert all that waste into safe nitrates.

How Do I Increase My Biological Filtration?

We naturally want to know how we can ensure enough biological filtration to manage toxic nitrogen compounds in our aquariums. One easy way is to of course add more aquarium plants, which will happily consume the ammonia and nitrates produced by your fish’s waste. Just remember that if you don’t have enough fish waste to feed your plants, they could starve to death, so you’ll need to supplement with a good, all-in-one fertilizer like Easy Green.

It is common to believe that purchasing more filters will increase the number of beneficial bacteria in your aquarium. Beneficial bacteria can grow on all surfaces in an aquarium. This includes glass walls, gravel, decorations, and even glass walls. Buying more filtration simply means you have greater capacity to hold more beneficial bacteria, but if you only have a few fish, your decor alone may have enough surface area to colonize the necessary beneficial bacteria.