The Easy Guide to the Nitrogen Cycle for Aquariums

Are you getting started with your first fish tank? Perhaps you have heard of the “aquarium cycle,” which involves complex graphs and scientific terms that can be 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.

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.

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.

Second Step: Beneficial Bacteria #1 eats 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. The best way to get rid of nitrates is to change the water or use aquarium plants. (Aquarium plants can use the nitrates in order 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. 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.

What is the average time it takes for an aquarium to cycle?

It can vary depending on the situation, but it usually takes between a few weeks and 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). You can read the complete article about 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. 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 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.

There is a common myth that larger or more filters will result in more beneficial bacteria. 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.