Freshwater Fish Tank Cycling – how to Prepare for New Fish

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Freshwater Fish Tank Cycling – How to Prepare for New Fish

You have probably never seen fish in wild, living in clear, pure water that is free of any contaminants. Probably not. 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 Nature’s method of reusing nutrients is called the

nitrogen cycle

. At a very high level, bacteria and plants convert and consume nitrogen compounds from their environment, animals eat the plants and bacteria, and then the plants and bacteria consume the animal’s waste as their 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. But wait! What happens if you’ve just set up a new aquarium with tap water, gravel, and decorations? Where are the plants and beneficial bacteria that can break down the fish waste? 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 approach is the most common, used by both beginner and veteran fish keepers alike. Most people cannot watch an empty aquarium for several weeks or months, hoping that beneficial bacteria are actually growing. So, here are a few tips to help you get started on the right foot:

– 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. Start slowly with your fish and increase gradually over the next four to six week. The beneficial bacteria feeds on fish waste, but since there isn’t a lot of bacteria in the beginning, you don’t want to overfeed the animals until enough bacteria has grown to handle their waste load. – 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. Live nitrifying bacteria can be purchased to speed up the cycle.

– Measure the water quality using ammonia test strips and multi-test strips; this should be done every day or two at first. Whenever you see a detectable amount of ammonia or nitrite (anything above 0.2 ppm), do a partial water change to remove the toxic compounds and provide new, clean water for your fish.

– The cycle is considered “complete” once you are able to feed your fish normal amounts of food for a week, and ammonia and nitrite levels stay at 0 ppm while nitrate levels are above 0 ppm. You can add fish slowly, but with some waiting time – to make sure the beneficial bacteria growth continues to grow despite the increased waste load. Although nitrate is considered safer for fish than other compounds, it should be removed from the water supply once it reaches 40 ppm.

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. Microbiologist Diana Walstad says that aquatic plants actually consume nitrogen waste more efficiently than bacteria. All the more reason to go for a planted tank! (That said, the leaves and roots of the plants will contribute some beneficial bacteria, but you can add even more using the tips mentioned above.)

Once the plants (or algae) show new growth, the cycle is complete. 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.

Planting a tank in a garden is beautiful and can improve the 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. This process is not recommended for beginners. We’ve seen it fail many times and have had to help others in the hobby.

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.