This guide explains the importance of the nitrogen cycle, including a step-by-step fishless cycling, what to do if you are already doing a fish-in cycle, and some frequently asked questions.
Setting up a new aquarium is exciting, but before adding any fish there’s one crucial step every aquarist must complete: cycling the tank. Cycling establishes the beneficial bacteria that make an aquarium safe for fish by processing harmful waste products.
Fish produce waste that releases ammonia, a toxic compound that can quickly build up in a new tank. In a healthy aquarium, naturally occurring bacteria convert ammonia into nitrite, which is also toxic, and then into nitrate, which is far less harmful and can be removed through regular water changes and plant growth. This biological process is known as the Nitrogen Cycle.
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Why do we need to cycle a fish tank?
In a brand-new aquarium, the bacteria to process fish waste are not yet present in sufficient numbers. Without them, ammonia and nitrite can rise to dangerous levels within days harming, and potentially killing, your new wet pets. Cycling the aquarium allows these beneficial microbes to colonise the filter media, substrate and other surfaces in the aquarium before fish are introduced.
Most aquariums can often take four to six weeks to cycle fully, although the exact timeline can vary depending on numerous factors. Temperature, filter efficiency, water chemistry and whether established media or bottled bacteria are used all influence the cycle length.
The safest method is the fishless cycle, which uses a controlled source of ammonia to feed the bacteria without exposing fish to toxic conditions. This is the method we at Practical Fishkeeping recommend, however fish-in cycling is sometimes recommended to new aquarists. Therefore, we will also cover how to do this in the safest way in case you are already going down this route.
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What is fishless cycling
This method of cycling a new aquarium mimics the presence of fish by using household ammonia (typically ammonium chloride) or another waste substitute. Feeding an empty tank fish food can work, but it is not as accurate and can produce the wrong types of bacteria, decomposing heterotrophs.
The main benefit of fishless cycling is that we can grow a healthy population of microorganisms, like autotrophic bacteria and archaea, without exposing any fish to high levels of toxic waste. Plus, it really isn't that difficult if you follow these simple steps.
Fishless cycling shopping list
- A quality test kit, we like the NT Labs Aquarium Lab as it includes a test for KH as well!
- Ammonium chloride solution - Dr Tims is an aquarium specific product but some DIY stores may sell household ammonia. Just check it does not contain any other chemicals!
- Bottled bacteria - most aquarium companies will have their own version, NitriCo Goop seems the best as it is packaged fresh.
Fishless cycling step by step
- First, you need to know the volume of your aquarium. Check the manufacturers website for the stated volume of measure the tank in centimetres. Times the length, width and height together and divide by 1000 to get your volume in litres. Reduce this volume by about 10% to account for the aquarium decoration unless a large external canister filter or sump is being used, in which case they likely balance each other out.
- Use an online calculator to work out the correct dosage of ammonia for your tank. We typically recommend aiming for a concentration of 3ppm ammonia in your tank.
- Test the water daily to check for a decrease in the ammonia readings. Use the calculator again to work out how much ammonia is needed to bring the levels back up to 3ppm.
- In a few days you can start also testing for nitrite, the next chemical in the nitrogen cycle. Once you are seeing nitrite being produced you know you are on the right track, but keep dosing the ammonia to 3ppm whenever it is required.
- Test the pH of the water routinely during this process, a sudden drop in pH may mean that the bacteria are using all of the carbonate ions (which buffer the pH). This is especially common in soft water areas. If this occurs, you can either do a large water change and redose the ammonia, or add a KH/alkalinity buffer to increase you carbonate ion concentration. The nitrogen cycle will stall if you allow the pH to crash as the bacteria stop working in acidic conditions.
- You will start seeing nitrate levels increase if you begin testing for this. This is the final product of the nitrogen cycle which will need removing by plant growth and water changes. This is a good sign that your tank is almost there.
- Continue to test and dose ammonia until the 3ppm of ammonia becomes zero within 24 hours. This means you have a healthy colony of bacteria. However, you must keep them fed with ammonia until you add fish!
- Once you are ready to go fish shopping, do a large water change to dilute all those nitrates that have accumulated. You can now safely add your new fish, but don't go crazy and add the full stocking of your aquarium in one go. Instead, take it slow and build up the fish stocking gradually.
Photo by Matt Hayes
Fish-In cycling
Some websites and retailers may recommend fish-in cycling as an easier method for beginners. While it does require less additional purchases (you don't need ammonia) it is still very labour intensive. Plus, if it goes wrong it can result in your fish dying which is a good enough reason for us to avoid it.
However, if you have already begun fish-in cycling before reading this, there is no going back. You now have fish in your aquarium and it is now critical you follow this guide to ensure your fish remain healthy during the fish-in cycle.
Just like with the fishless cycle, you will need a good quality test kit to check your water quality at least daily. If the ammonia or nitrite levels reach 0.1ppm you will need to do a large water change of at least 1/3 the tank volume (although larger is often better).
Here lies the problem, this water change prevents your fish from harm, but it removes the food source your bacteria need to grow and reproduce! This means it takes a lot longer for your aquarium's filter to fully mature.
Have to hand a water conditioner that detoxifies ammonia to hand, something like NT Labs Optimus, or Seachem Prime. This can help prevent fish losses when the concentrations of the toxic compounds spike, although it should not be used instead of a water change.
Fish-in cycling shopping list
- Ammonia detoxifier - Prime, Optimus, or Ammo Lock
- A quality test kit, we like the NT Labs Aquarium Lab as it included tests for KH as well!
- Bottled bacteria - most aquarium companies will have their own version.
FAQs on fish tank cycling
Can I cycle a tank immediately with a bottled bacteria product?
In short, no. Using bottled bacteria cannot be relied upon to full cycle your aquarium quickly, and you may find yourself stuck with a fish-in cycle if you attempt it.
However, these products can be really helpful speeding up the process of a fishless cycle by introducing the bacteria we are trying to grow. If you are carrying out a fish-in cycle they are also worth having, as they can help speed up the process.
Can I add mature filter media from another aquarium?
If you are setting up another aquarium you can certainly help your new tank along by using some mature filter media. There are a few things to consider first though.
- Is there anything in your existing fish tank that you do not want in your new aquarium? Pest snails and disease can easily be transferred via the old filter media.
- Only take a small amount of the established filter media, as you are removing the bacteria that are converting your fish's waste. Increase water testing of the established aquarium after removing the established filter media to check for ammonia spikes. Alternatively, squeeze the foams from the old tank into the new one to transfer some bacteria without removing a large chunk of it. This will be no different than what you do when you clean the filter foams but can help speed up the process for the new tank.
How long does it take to cycle a fish tank?
This is a question that every aquarist probably asked when first starting in the hobby. Unfortunately the answer is still not definitive as every aquarium is different. Expect a fishless cycle to take at least a month, but there are so many variables that it is impossible to give a definitive answer. The important thing is to stay patient and not to rush it.
Why has my cycle stopped?
If everything was going to plan, nitrite and then nitrate levels began to increase but suddenly the ammonia is no longer dropping, it could be that your KH has dropped too low. The bacterias involved in the nitrogen cycle use carbonate ion in the process, lowering your alkalinity, or KH.
These ions are not only needed for your bacteria to function but they also play a role in buffering the pH. Without carbonate ions you will see a sudden drop in your pH levels in the aquarium. At low pHs (below 6) the bacteria effectively stop working altogether.
If you are carrying out a fish-in cycle and notice your cycle crashing, your fish may still look healthy. This is because in low pH environments all the ammonia is present as the less toxic ammonium ion.
This is a ticking time bomb though, as a small water change may raise the pH sufficiently for some of the ammonium to convert back to ammonia and potentially kill your fish. In this instance you need to be very careful. Use and ammonia detoxifying product along with water changes and/or KH buffers to bring the pH and KH up while preventing harm to your fish.
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