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How to Cycle a Fish Tank
Last updated on November 17th, 2023
Nitrogen Cycle for Fish Tanks
The purpose of this guide is to teach you how to cycle a fish tank. Both fresh and saltwater fish tanks need to cycle in order to build the beneficial bacteria responsible for removing ammonia and nitrites from the tank, and converting them to the much less toxic nitrate. There are a number of ways to initiate the cycle in a fish tank, and I will show you which ones I recommend and how to know when the cycle is done.
What is the Nitrogen Cycle in Aquariums?
Before learning how to cycle a fish tank, you first need to know a little bit about what the nitrogen cycle does for the aquarium.
When fish food or waste breaks down, it releases ammonia into the tank. Ammonia is very toxic to fish, as well as most life in a fish tank. Even small amounts of ammonia has the ability to kill most fish. The first thing that happens when you cycle a fish tank is that you grow the nitrosomonas bacteria colonies that convert ammonia to nitrite.
Though still toxic to most fish tank life, nitrite is far less toxic than ammonia. When you cycle a fish tank, however, you don't when nitrites are present. The second step when you cycle a fish tank is building another beneficial bacteria, nitrobacter, that converts nitrite to the far less toxic nitrate. Nitrate, though toxic in large quantities, is nowhere near as toxic as nitrites or ammonia is the fish tank.
Nitrate is then consumed by algae, plants, and coral to grow. These may then be consumed by fish and other aquarium life, or simply die and decompose. Both of these then provide ammonia to the fish tank and starts the nitrogen cycle again. If you are patient and build a proper bacteria colony when you cycle your fish tank, you will never see ammonia and nitrite before it is converted to nitrate.
When you finish cycling a fish tank, the beneficial bacteria colonies will be able to convert ammonia to nitrates before they are even detectable. Basically meaning rotting food and waste will seem to break down into nitrates rather than ammonia. The info-graphic above illustrates how the nitrogen cycle works at a high level.
If you are interested in more information about the Nitrogen Cycle in Fish Tanks, read the article below where I cover this in more detail.
How to Cycle a Fish Tank
Now that you have a high level understanding of what the nitrogen cycle is, How do you cycle a fish tank? It's easier than you think. Once you add an ammonia source, and enough surface area for the beneficial bacteria to grow, the nitrogen cycle handles itself. You will need to monitor the cycle process to know when it has finished. Other than that, however, you will need to be patient and avoid trying to take short cuts, or speed up the fish tank cycle.
When you cycle a fish tank, it is possible for the cycle to take up to a few months. There is no defined time because every fish tank will be different. I repeat, Do not try to rush the cycle. Methods such as using chemical additives, doing water changes, or any other "cycle a fish tank fast" tricks people may suggest can actually cause less bacteria to grow and not be sufficient to support your fish tank. These short cuts often end in disaster and cause ammonia and/or nitrite spikes later, killing your fish.
Make Sure You Have Enough Surface Area
Before you cycle a fish tank, you need to make sure you have enough surface area for beneficial bacteria to grow. Common ways to increase your surface area include:
- Fish Tank Substrate
- Saltwater fish tanks tend to use sand, while Freshwater tends to use gravel as a substrate for the aquarium. Both of these provide a large amount of surface area for beneficial bacteria to grow.
- Decorations and Rock
- Porous rocks provide a lot of surface area for beneficial bacteria to grow. For saltwater fish tanks, live rock usually solves much of this problem. Many prefer decorations in their fish tank, which contains far less surface area. No worries, there are ways to increase the surface area while still using decorations.
- Refugiums
- Many fish tanks, particularly saltwater fish tanks, use refugiums. The refugium is usually in the sump, containing large amounts of sand and rock for beneficial bacteria to grow on. These are usually found in a section of the sump, but can be as advanced as an additional tank.
- Biological Filtration Media
- Often times, the above methods don't provide quite as much surface area as you may want. To help with this, there are a number of biological filtration media options available. Here are some of the ones I recommend.
Adding an Ammonia Source
When adding an ammonia source to cycle a fish tank, you'll find a variety of methods. Some of these ammonia sources are better than others. Let's look at how some of these work, why they may or may not be recommended methods for how to cycle a fish tank.
Cycling with Fish - NOT RECOMMENDED
Far too often, people recommend adding hardy fish to the aquarium to start cycling the fish tank. Please, DO NOT DO THIS. Do not add fish to a tank that you know is a toxic environment for them.
When you cycle a fish tank by adding fish, you are risking the life of the fish. The ammonia spike is very likely to kill the fish, or at very least cause health problems such as gill burns and illness. Not only will you lose money by killing fish, which can get expensive quickly for saltwater fish tanks. You will also have a sad and unsuccessful feeling when the fish get sick and/or die. Please DO NOT cycle a fish tank by adding fish.
Cycling with Fish Food - Ghost Feeding
A very easy way to begin the cycle in a fish tank is to just add fish food. Adding food for the first day or two, feeding as if there were fish in the tank, will be enough to start the cycle. The food will break down and start releasing in to ammonia, starting the cycle.
Cycling with Ammonium Chloride
Rather than waiting for the fish food to break down, you can add ammonia to the fish tank directly. When you cycle a fish tank, do not use regular home cleaning ammonia, but rather a product such as Dr. Tim's Ammonium Chloride.
This will supply the fish tank with the ammonia needed to safely start the nitrogen cycle. You can also add a starter colony of beneficial bacteria using Dr. Tim's One & Only Nitrifying Bacteria. Though I have not used these products myself, they are heavily supported in the fish tank community.
Measure and Track Parameters
At this point, you will want to measure and track your Fish Tank Cycling based Parameters. When you cycling a fish tank, it is important to monitor Ammonia, Nitrite, and Nitrate levels. You should test these three parameters two or three times per week, until the cycle is complete. This will help you identify and learn where you are in the cycle.
There are a few test kits that are better than others. For more delicate parameters, better test kits are recommended. For ammonia, nitrite, and nitrate, you don't need anything too high end. That being said, I still recommend the salifert test kit. However, if you are looking to save about $25, the API Master Kit should be fine for tracking the cycle.
- Saliftert Nitrate Test Kit
- Saliftert Nitrite Test Kit
- Saliftert Ammonia Test Kit
- Red Sea Marine Care Test Kit (Nitrate, Nitrite, Ammonia, pH, and Alkalinity)
- API - Freshwater Master Kit
- API - Saltwater Master Kit
What to Expect when you Cycle a Fish Tank
When you cycle a fish tank, and track the parameters, you will see things get worse before they get better. You will see ammonia continue to rise as the food rots or ammonia chloride is added. This will rise or plateu until the beneficial bacteria that converts ammonia to nitrite populates. Once the beneficial bacteria is present, you will start to see nitrite rise. As the nitrite rises, you will see the ammonia drop.
Much like with ammonia, you will start to see nitrites rise until the nitrobacter beneficial bacteria populates enough to handle the nitrite. One the bacteria colonies grow, the nitrites will drop and nitrates rise. When this happens, you are starting to get close to the end of the cycle!
When is the Fish Tank Cycling Done?
When the ammoniaand nitrite are both undetectable, or 0, your fish tank is cycled. Some people prefer to wait fot the nitrate levels to remain constant for about a week. This adds extra confidence that the fish tank is ready for fish!
At this point, you should be able to add one or two fish. Keep monitoring ammonia, nitrite, and nitrate to make sure these level do not spike. Add only a couple fish for the first month to let the fish tank level out. Always add livestock slowly to prevent spikes in these parameters. If you add a large number of fish and invertabrates all at once, you will likely cause a spike in ammonia and/or nitrite, causing more problems,
This video from Marine Depot provides additional information about how to cycle a fish tank.
What to Watch For
When you cycle a tank, after adding fish, it is possible that the ammonia spikes. This usually means you don't have enough surface area for the bacteria. If this is the case, you will have to add more surface area to allow for more bacteria to grow. If this is the case, I ecourage you to check out bio bricks and bio plates because they add a large amount of surface area in a small total space in the sump.
Here are the solutions I would look at:
- Brightwell Aquatics Xport BIO - Plate
- Brightwell Aquatics Xport BIO - Dimpled Brick
- Brightwell Aquatics Xport BIO - 2 Inch Blocks
Alternatively you may not have let the cycle finish. If this is the case, be patient and wait it out. If you add fish before the tank is ready, it will cause ammonia and nitrite to rise and likely kill or seriously harm the fish.
Conclusion
This guide showed you how to cycle a fish tank, and helped you start your fish tank adventure. This part of keeping a fish tank can be difficult because it is all about patience. All we want to do while looking at an empty fish tank is to start adding fish. I promise, being patient and waiting will set you up for much more success and you will not regret it!
Additional Reading:
- Nitrogen Cycle in Aquariums
- Reef Stable's Guide to Saltwater Fish Tank Filters
- Reef Stable's Guide to Micro Bubble Scrubbing
Frequently Asked Questions
How long does it take for a fish tank to cycle?
When you cycle a fish tank, it can take up to 3 months. Though some fish tanks can cycle in as little as a month, many take as many as 3 months to finish cycling. Be patient, take it slow, and you will have better results.
How do I know when my fish tank is cycled?
When you cycle a fish tank, ammonia will rise, then begin falling. As ammonia falls, nitrite levels will rise. You will then notice nitrites fall and nitrates rise. Your fish tank is cycled when both ammonia and nitrite remain at 0, and nitrates are level, for roughly a week.
Do you need fish to cycle a tank?
No, you do not need fish to cycle a fish tank. Though it is possible to cycle a tank with fish, it is not recommended as it can be deadly for the fish. There are a variety of fish-free methods to cycle a fish tank.
How do I know if my tank is ready for fish?
When you cycle a fish tank, ammonia will rise, then begin falling. As ammonia falls, nitrite levels will rise. You will then notice nitrites fall and nitrates rise. Your fish tank is cycled when both ammonia and nitrite remain at 0, and nitrates are level, for roughly a week. At this point, your tank is ready to slowly add fish. Start with one or two for the first month to ensure the fish tank is stable.
About the Author
John Krenzer
John is a Software Engineer with a passion for saltwater aquariums, as well as the founder and president of Reef Stable. He started in the aquarium hobby as a child with a 20 gallon freshwater aquarium. His interest in aquarium life grew and in 2008, John set up his first saltwater aquarium.
Today, John maintains an over 300 gallon reef tank system, consisting of a 120g reef and a 210g reef. These large tanks are contained within the same system, sharing a sump as a means to reduce total maintenance and increase total water volume.
John writes articles for the blog as a means to learn about more reef aquarium topics. These articles act as a reference for the readers as well as himself. John updates these articles frequently to provide additional information or make corrections as new information becomes available.
If you would like to request an article, tank tour article, or to collaborate, let me know via the Contact Me Page!
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