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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 for 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.

A simplified diagram of 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.

For the purposes of our illustration, let’s use yellow, brown, and blue M&M’s to represent 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 then eats the nitrites and produces nitrates (the least toxic nitrogen compound).


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 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. 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. If the nitrates reach 40 ppm or more, then you need to remove some of the dirty tank water and replace it with fresh, clean 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. You can speed up this process by buying a bottle of live nitrifying bacteria, getting some used filter media from a friend, or growing live plants (which also come with beneficial bacteria on them). For more details, read the full article on how to cycle your 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. In reality, the answer is a little 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 Do I Increase My Biological Filtration?

This naturally leads us to ask how to we make sure there’s enough biological filtration in the aquarium to handle 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.

As for growing beneficial bacteria, there is a common misconception that buying bigger or more filters will increase the amount of bacteria in your aquarium. The truth is that beneficial bacteria grows not only in filters but also on every surface in your aquarium, such as the gravel, glass walls, and decorations. 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.

To find out how often you need to do water changes on your aquarium, download our free infographic that guides you step-by-step through the process.

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