How to Cycle Your Fish Tank

LAST UPDATED 13th of March 2023

This article is written by Pet Circle veterinarian, Dr Emilee Lay BSc (Vet) Hons BVSc and edited by Dr Nicole Wynne Bsc BVMS MANZCVS (Unusual Pets)

The first few months of starting a new fish tank are the most daunting, especially for new fish owners. Tank preparation and establishing a healthy tank and water system can take up to 3 months before fish are even added to the tank, but the patience and diligence will pay off, as a properly cycled and established tank has a much lower risk of developing new tank syndrome, and your fishes’ health relies directly on a stable water system!

The terminology can also be confusing, so here’s a rundown of common fish tank terms:

  • Tank cycling: The process of setting up a brand new fish tank and establishing colonies of microscopic bacteria in the tank and filter system that is instrumental for breaking down fish waste.
  • Filter: Any mechanism that helps to reduce waste in water, not necessarily just the machine! There are three types of filters Chemical, Biological, and Mechanical. 
  • Water conditioner: A chemical additive that binds some toxins like ammonia and chlorine in the water, and that may also contain biological filter. Despite the claims of some products, water conditioners are not a substitute for proper tank cycling or water maintenance. 
  • Water testing: Usually done with a kit, water testing identifies the levels of particular chemicals in the water that can affect water quality, and therefore fish health. There are several chemicals and other parameters that should be tested for including Ammonia, pH, Nitrate, Nitrites, GH and KH. 

Once you have an idea of what kind of plants (real or fake), filters, gravel, decorations, heaters, air pumps, and accessories you want for your tank. It's time to start cycling! Check out our article on Top 10 Products You Need to Set Up Your Aquarium 


Fish In versus Fishless Cycling

As exciting as it can be to add fish straight away to your fish tank, your tank needs to be appropriately cycled in order to safely add fish in. Cycling involves establishing a healthy bacterial population that can regulate the wastes/nutrients in your water aka the nitrogen cycle. It can be done in one of two ways - Fishless and Fish-in Cycling. 

Fish-in Cycling was the more common approach where sacrificial fish were used to help kickstart and establish the nitrogen cycle. However, it can be far more challenging to establish a successful cycle, and without careful monitoring, these fish are most likely to die. It generally involves slowly adding fish over several weeks, frequent water testing and minimal feeding to encourage cycling, and frequent water changes dilute any excess ammonia on nitrites. "Fish in Cycling" is not recommended for beginners. The welfare concerns with Fish-in Cycling are also an important drawback.

On the other hand, fishless cycling has none of these risks. It’s great for beginners and experienced aquarists alike. The main difficulty is being patient while the cycle is being established. The main goal for fishless cycling is to seed the filter with enough beneficial bacteria to cope with waste by adding in ammonia directly OR using a source of waste such as fish food or raw prawns. 

Understanding the Nitrogen Cycle

As your fish poops and waste is generated in your aquarium from uneaten food and rotting plants, the breakdown of these products releases ammonia into the water. Ammonia is highly toxic to fish. Beneficial bacteria known as nitrifying bacteria will naturally appear in the tank when being cycled, and they start to break down this ammonia, eventually consuming it faster than it is being released

In the second stage of the cycle, nitrites begin to rise as a by-product of the nitrifying bacteria. Nitrites are also highly toxic to fish and a second type of nitrifying bacteria that consumes ammonia appears to help reduce the nitrite levels. 

In the final stage of cycling, the nitrite levels begin to decline and the nitrate levels are on the rise. This second bacteria in the process of consuming nitrites releases nitrates. Nitrates are mostly harmless in small concentrations but can be toxic in large quantities. Regular water changes can then help regulate the nitrates in your aquarium. 

Your aquarium is cycled when the population of these two beneficial bacteria is at a point where they can consume ammonia and nitrites as quickly as they are produced. 

Fishless Cycling

What Equipment Do I Need To Start Cycling My Tank?

Before cycling your tank make sure you have the following equipment: 

  1. Freshwater Test Kit 
  2. A Source of Ammonia (Either as Ammonium Chloride OR fish food/raw prawns)
    1. Pure ammonium chloride can sometimes be purchased from a specialty aquarium to kick start a fishless cycle. Do not use household products - they may contain scents and other chemicals that can kill fish. Small amounts of fish food or raw prawns can be used as an alternative. 
  3. Dechlorinator/Water Conditioner 
    1. Chlorines and chloramines in water kill beneficial bacteria, it is always essential to use a water conditioner when adding water to your aquarium, as well as leaving water to sit uncovered for 24 hours to allow chlorine to evaporate. 

Set up your tank ensuring the heater, filter, air pump, substrates, plants and decorations are added in and running. Please note that bacteria prefer a temperature of 18 C to 30 C and temperatures outside this range can slow or inhibit growth. 

How Long Does it Take to Cycle a Tank? 

Every tank is different and it can take up to 6-8 weeks for a tank to be completely cycled. This is why forward planning for fish is a must to allow you the time to cycle your tank, leading to happy and healthy fish.

What about Chemicals that Allow Me to Add Fish Straight Away?

It's important to realise that chemicals that claim to negate New Tank Syndrome and allow for fish to be added directly to a new tank, should be used cautiously. Products like these simply contain beneficial bacteria needed as part of the cycling process. These products may potentially speed up the process of cycling BUT any fish added will be subject to fluctuating water parameters despite these claims. In reality, you will still need to wait whatever length of time for your tank to be fully cycled. 

Step 1. Testing Your Tap Water and Adding a Water Conditioner

This is often a crucial step to understanding what your normal tap water range is. Testing your tap water will give you an idea of the pH of your tap water. Tap water will vary within a metropolitan range as sources may come from different catchments or travel through different pipes. Reading your pH, ammonia, nitrate and nitrite levels can help you navigate any water quality issues you may have even before cycling your tank. 

Although ammonia, nitrate and nitrite is the prime focus of cycling, pH is another parameter that can influence the cycling process. A pH below 7 or above 8 is not ideal for most species of fish; however, there are some specialised species that prefer acidic water. Try and raise your pH if your regular tap water has a pH below 7 with a commercial pH raiser (most tap water should sit at 7 and above!). Monitoring your tank water as it cycles is crucial. If your pH drops below 7 during the cycling process, a water change may be needed.

It is normal for tap water to have some nitrate as well - this is important to note down so that you can see what your “baseline” is whilst cycling. Once you have tested this, fill your tank and add the appropriate dose of Water Conditioner. 

Water conditioners neutralise chlorine, chloramine, and any metal toxins in the water which can be harmful to fish. There are several tap water conditioners on the market, some of which will also help to detoxify ammonia and nitrites or have additives to reduce stress.

When picking a water conditioner consider the size of your tank, some are quite concentrated and are perfect for large-volume tanks which may not be ideal for smaller tanks. Always remember when adding water conditioner to follow the directions accordingly. Going forward anytime you do a water change at home, you will need to add a water conditioner. 


Step 2. Adding Ammonia

You need a source of ammonia to kick-start tank cycling. Ammonium Chloride can sometimes be purchased from a specialty aquarium so that you can tailor the ammonia levels in your tank whilst cycling. Otherwise, you can substitute this at home by using fish food or a raw prawn in a mesh bag. 

With the Fish Food Method, fish food is slowly added to the tank, creating ammonia as it breaks down. You can also use a fine mesh bag containing fish food in the tank. You are aiming to have ammonia levels of at least 2-3ppm (parts per million) to kick start the cycling process. With the Fish Food Method, regular testing of ammonia is needed to ensure you are reaching adequate levels in your tank water. This does involve a bit of trial and error and you will need to test daily for it during the cycling process. If the ammonia levels drop below 2ppm, you will need to add in some food or ammonia. Make sure to try and test ammonia at a similar time each day, throughout the cycling process. 

Start testing for nitrites daily after 1-2 weeks. You may notice the ammonia levels start to drop as the nitrite levels start to climb. Ensure that the ammonia levels remain under 5ppm but not 0ppm. You may need to add half the normal amount of fish food to help maintain this level. 

Step 3. Monitoring Nitrites and Nitrates 

Over time the ammonia and nitrite levels will drop quite rapidly as the second type of nitrifying bacteria begins to develop. This step can take several weeks. Once nitrite levels start to drop, test for nitrates. The final stage of cycling is marked by the presence of nitrates. Continue to test for ammonia, nitrites, and nitrates daily, and add ammonia every couple of days to help keep the cycle going. The cycle is complete once nitrates are being produced while ammonia and nitrites show a 0ppm reading 24 hours after adding ammonia to the tank. 

Recording your results each day will help you keep a close tab on the cycling process as well as any issues that may arise. Before you add any fish to your tank, do a partial water change to remove the nitrates that have built up. If there is some time between when your tank has finished cycling and when you add fish, you may need to add in a daily source of ammonia so that the beneficial bacteria don’t starve!


Adding Your Fish

Your tank is fully cycled, and it's time to add your fish, however, the maintenance and care doesn't end there. Once you've added your fish - following our Guide To Introducing Fish to Your Tank - you will need to monitor the water parameters for the first few days. You can also continue to add beneficial bacteria to help boost the biological filter of your tank. Check out our guide to Common Aquarium Problems to learn more. 

Further Reading

Want to read more? Check out our other articles:

Fish Tank Maintenance and Cleaning 101

A Guide To Fish Filters

A Complete Guide to Setting Up Your Fish Tank

Common Aquarium Problems and How to Solve Them

Feeding Your Fish

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