Freshwater Fish Tank Cycling – how to Prepare for New Fish

Freshwater Fish Tank Cycling – How to Prepare for New Fish

Do you remember ever seeing a wild fish swimming in crystal-clear water without any other contaminants? It’s unlikely. That’s because life isn’t sterile; it doesn’t flourish in “pristine” conditions, but rather when there’s a whole ecosystem of microorganisms, plants, and animals that are in balance with one another. So, if you’re looking to set up a brand-new fish tank, let’s talk about aquarium cycling and how to prepare a healthy, thriving environment for your fish to live in.


What is the Aquarium Nitrogen Cycle?

The ability of nature to recycle nutrients is called the

nitrogen cycle

. Plants and bacteria convert nitrogen compounds to plants at an extremely high level. The animals then eat the bacteria and the plants then eat the bacteria’s waste as food.

In an aquarium, the same thing happens. Fish produce waste from eating, which is also known as ammonia. Once the waste has been absorbed by bacteria and plants, the water becomes safer for fish to drink. Wait! What happens if you’ve just set up a new aquarium with tap water, gravel, and decorations? Are there beneficial bacteria and plants that could help you break down the fish wastes? That’s why we need aquarium cycling: the process of making sure that your fish tank’s ecosystem can process ammonia without killing any animals.

For more details about how the aquarium nitrogen cycle works, check out our full explanation here.

How to Cycle Your Aquarium

There are several ways to cycle an aquarium, and some are easier than others. These are the best methods we have found to work based on our experience with hundreds of fish tanks.


Fish-In Cycling

This is the most popular approach, and it’s used by both novice and experienced fish keepers. Most people cannot watch an empty aquarium for several weeks or months, hoping that beneficial bacteria are actually growing. These are some tips to help get you started.

– When setting up your aquarium, only add a few fish at the start. Some people recommend one small fish per 10 gallons. Take a look at your fish stocking and select the most hardy, durable species that you intend to keep. You should start slowly and gradually increase the food intake over the next four to 6 weeks. Beneficial bacteria feeds on fish waste. However, there isn’t much bacteria at the beginning so don’t overfeed them until there is enough. – You can significantly speed up the aquarium cycling process by adding beneficial bacteria from the onset. If you already own several aquariums (or have a friend that does), simply transfer some used filter media or substrate from an established fish tank to your new one. To speed up the process, you can buy live nitrifying bacteria.

– Test the water quality with ammonia test strips or multi-test strips. This should be done every other day for the first few days. Do a partial water alteration if you find ammonia (or nitrite) above 0.2 ppm. This will remove any toxic compounds and provide clean water for your fish.

When your fish can eat normal food for one week, ammonia or nitr will be considered complete. Nitr and ate level are both above 0ppm. (At this point, you can begin slowly adding more fish, with some wait time in between – just to ensure the beneficial bacterial growth keeps up with the increased waste load.) While nitrate is safer for fish, it can cause nitrate concentrations exceeding 40ppm. This means that it is time to change the water to reduce the levels.

A water test kit helps you determine if there are toxic levels of nitrogen compounds in the aquarium.

Cycling with plants

This method is our favorite because it truly transforms your aquarium into a natural ecosystem, both biologically and visually. Rather than setting up a bare tank with very little to no fish, you can immediately add live aquarium plants and then focus on growing them with good lighting, substrate, and fertilizers. In fact, according to microbiologist Diana Walstad, aquatic plants consume nitrogen waste even more effectively than bacteria. This is why you should plant a tank. You can also add beneficial bacteria to the roots and leaves of your plants by using the tips above.

The cycle ends when the plants or algae show new growth. Your plants are successfully consuming ammonia and nitrates and converting them into new leaves and roots. Start adding a few fish to your aquarium. Then, use the water test kit and check that ammonia, nitrites, and nitrates, are below 40ppm.

A planted tank is not only beautiful to behold but also improves water quality for your fish.

Fish-Less Cycling

This technique for cycling has gained a lot of interest on the Internet, and it involves placing fish food or other source of ammonia in an empty aquarium to grow bacteria. After helping countless people in the hobby, we don’t recommend this process for beginners, since we find that many new fish keepers typically do it wrong and struggle to complete the process.

If you are determined to use this method and you know what to do, you should seed your tank with beneficial bacteria using used filter media or a bacteria additif. Otherwise, you will have a long road ahead.

Final Thoughts on Cycling

Aquarium cycling requires a bit of effort (and patience) on your part, but trust us – the results are totally worth it. By preparing a welcoming ecosystem for your new fish, you greatly minimize loss of life and make your aquarium maintenance routine easier.