The Easy Guide to the Nitrogen Cycle For Aquariums

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The Easy Guide to the Nitrogen Cycle for Aquariums

Are you getting started with your first fish tank? Then you may have heard of something called the “aquarium nitrogen cycle,” followed by a bunch of complicated scientific terms and graphs that seem a little overwhelming. No need to panic! Keep reading as we explain the nitrogen cycle in this very short and simple guide.

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.

Here is a simplified diagram showing the nitrogen cycle in 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 can remove nitrates by changing the water or using aquarium plants. (Aquarium plants use the nitrates as fertilizer to grow new leaves.


“Cycling your aquarium” simply refers to the process of making sure you have enough biological filtration (e.g., beneficial bacteria and aquarium plants) so that all the ammonia and nitrites get eaten up right away. Multi-test strips or ammonia test strips should be used to measure the ammonia and nitrite levels 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 all depends on how long it takes, but typically it takes anywhere from a few months to a few months. This can be accelerated by purchasing a bottle of live bacteria, obtaining used filter media from someone you know, or even growing live plants, which also have beneficial bacteria. Read the entire article to learn how to cycle an aquarium.

Most hobbyists will answer yes or no to the question of whether their aquarium is cycled. However, the truth is a bit more complex. 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 can I increase my biological filter?

So, how can we ensure that the aquarium has enough biological filtering to deal with toxic nitrogen compounds? 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. It is not true that beneficial bacteria grows in aquarium filters only. They also grow on the gravel, glass walls, decorations and other surfaces. 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.