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. There’s no need to panic. We will explain the nitrogen cycle in this short 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 consumes ammonia and makes 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 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 need to get rid of any dirty water from your tank and replace it by clean water if the nitrates exceed 40 ppm.

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). Read the entire article to learn how to cycle an aquarium.

Asking a hobbyist whether their aquarium has been cycled will result in a simple 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.

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. 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.