The Nitrogen Cycle and the Fishless Cycle – getting your aquarium ready for fish

Fishless cycling and the nitrogen cycle are something you’ll hear a lot about in relation to fishkeeping. It is arguably ‘the’ most important thing you’ll ever learn when it comes to keeping fish. Now, don’t panic, it’s not scary and scientific, it’s very easy and it’s really important for the fish. You don’t need a degree in chemistry but a little effort now will pay dividends in the long run.

It’s probably fair to say that there are two levels. One is the stuff you absolutely need to know, no two ways about it. The second is the more in-depth side of things. You can probably get away without knowing all about that side of it. But you do need to know about the basic stuff otherwise you and your fish won’t enjoy your hobby as much as you should. This guide will take you through the need to know stuff.

What is a fishless cycle?

A fishless cycle is the term used to describe the process of growing a colony of ‘good bacteria’ in your aquarium filter before any fish are introduced to the tank.  By adding a source of ammonia you can encourage the growth of the bacteria that process ammonia to nitrite, and nitrite to nitrate.  But before you start a fishless cycle you need to understand the process you are undertaking.

The Nitrogen Cycle – the need to know stuff

The nitrogen cycle refers to the process that takes place in the tank to convert fish waste from toxic to non-toxic. Fish excrete ammonia through their gills, and urinate and defecate in their water. The urine and waste produces ammonia as it decays. Bacteria convert ammonia to nitrite, more bacteria convert nitrite to nitrate. This is what’s known as the nitrogen cycle. The process is sometimes referred to as nitrification.

Ammonia is toxic to fish, at best it damages their gills and causes breathing problems, at worst it can kill. In the wild it’s dealt with by a variety of things; rainfall dilutes it, bacteria in the soil and surrounding environment process it and in the sea the rocks in the reefs are full of bacteria which all work to make these toxins safe.

How does ammonia get dealt with in your aquarium?

It’s all down to ‘good bacteria’, the same ones that deal with ammonia in the natural environment. The difference is that the bacteria don’t exist at first in a new aquarium as there is nothing for them to feed on. The fishless cycle process develops the colony of ‘good bacteria’ and means we can replicate a natural process to help us keep our fish safe from toxins.

All fish tanks need a filter of some sort to house the ‘good bacteria’. Think of a filter as being a ‘bacteria hotel’. Filters suck water in by means of a small pump or powerhead. The water then flows over the sponges/ceramic shapes (‘media’) inside. The good bacteria live on, and in, the media and as the water flows over them they process the ammonia before the water flows back into the body of the tank.

There is a certain type of bacteria that likes to ‘eat’ ammonia, called nitrosomonas bacteria. If there is ammonia present in the water this bacteria will start to develop and multiply until there is enough bacteria to ‘eat’ all the available ammonia. The bacteria turn ammonia into nitrite.

Great, so we’ve dealt with the ammonia, lesson over?

Not quite. Nitrite is also toxic to fish. It binds with molecules in the blood that oxygen should bind with. The oxygen can’t bind with the blood properly and the fish starts to suffer. Its organs are not getting the oxygen they need to function properly and it starts to show signs of brown streaks in the fins as the blood carries less oxygen and starts to look brown instead of red.

Happily for the fishkeeper there is another type of bacteria that likes to ‘eat’ nitrite, called nitrospira bacteria. These bacteria are quite happy to share the filter with other bacteria, the ones that ‘eat’ ammonia.

These bacteria take a while to develop but once their numbers are sufficient they can work fast, and they are great at teamwork. As one lot munches on the ammonia and turns it to nitrite the other lot are ready to munch on the nitrite. The nitrite is converted to nitrate. Nitrate is not toxic to fish, unless allowed to build up to significant levels. There are no bacteria that can live in a conventional filter which can remove nitrate. Keeping nitrate at acceptable levels is the job of the fishkeeper and is achieved by doing partial water changes. When you remove some of the old water to replace with fresh water you are also removing some of the nitrate build up. The fresh water should be lower in nitrates and so the nitrate level in the tank will be decreased.

In summary:

  • fish excrete ammonia
  • organic waste breaks down into ammonia
  • bacteria in the filter convert ammonia to nitrite
  • more bacteria convert nitrite to nitrate
  • you remove nitrate with water changes

There, job done, nitrogen cycle understood.

Can I put my new fish straight into my new tank today?

Sadly no. The bacteria need a few weeks to get established first, it generally takes between four and six weeks to complete a fishless cycle to get the bacteria ready to look after your fish. But don’t worry, while you’re busy growing your bacteria you’ll have plenty of time to get your tank set up just the way you want it, and research the needs of the fish you want to keep. Getting the layout right before you add fish is much easier than trying to arrange things with a whole lot of swimmy helpers!  And being fully up to speed on the requirements of your new fish will mean you shouldn’t have any last minute panics. This also gives you the chance to make sure all your equipment is working properly.

But I was told to set my tank up, leave it a few days and then I could add fish?

This is something we see posted a lot on internet forums; well meaning keepers have bought their new tank and got it set up having been told by someone (quite often a shop) that they can just leave it a few days and all will be ready. Unfortunately this is not correct. Leaving the tank to stand for a few days doesn’t achieve anything, there is nothing in the tank to encourage the bacteria to grow so your tank is no more ready for fish after a few days than it was when you set it up.

At one point it was considered normal practice to add fish immediately. Quite often goldfish were used as ‘sacrificial’ fish to get bacteria going in preparation for the real residents. Sometimes people are recommended to start with a few ‘hardy’ fish to get the cycle going. Now that we understand more fully what happens to a fish when it is used in this way it is considered extremely bad practice to use fish to cycle a tank. Of course, there will always be someone who did it and says it was ‘fine’. Regardless of what they think or say, the fish are very unlikely to have felt ‘fine’ during the process.

INJAF is strongly against using fish to cycle a tank, and we encourage you to undertake a fishless cycle to prepare your fishes’ new home so that is ready for them to move straight into and enjoy.

What happens to a fish during the cycle that means they’re not ‘fine’?

Ammonia can have several unpleasant effects on fish, symptoms of ammonia poisoning including:

  • Respiratory distress
  • Internal and external bleeding, reddening areas of skin caused by hemorrhaging from blood capillaries in the skin and internal organs
  • Erratic swimming, hyperactivity and excitability and twitching as the nerves become damaged
  • Trying to escape from the water by constantly jumping out
  • Mucus hyper-production causing a white clouding of the skin
  • If untreated, death

The above was taken from the following article on, please read the full article for more detailed information (don’t worry, you don’t need a PhD for this!).  There’s also some very useful and comprehensive information in Practical Fishkeeping’s ‘Frequently asked questions on ammonia’ article.

As mentioned earlier, nitrite binds with the blood and prevents oxygen from doing so. Symptoms include:

  • Lethargy
  • brown streaks in the fins and tail
  • tissue damage
  • secondary infections such as bacterial infections
  • If untreated, death

There is more information on nitrite poisoning in an article on, please read the full article for more details (again, no PhD required!).  Practical Fishkeeping also has a ‘Frequently asked questions on nitrite’ article.

A fish exposed to ammonia and/or nitrite can suffer long term problems even when the water conditions are improved.  The above articles from Practical Fishkeeping and give a clear explanation of the problems a fish will face if a fishless cycle is not undertaken.  By doing a fishless cycle you can avoid these unpleasant and potentially life-threatening effects on your fish.

What about the bacteria in a bottle, I was told this would prepare the tank in a matter of days?

Bottled bacteria causes a lot of debate on fishkeeping forums!  We’re working on some more detailed information about this so watch this space for more information. It’s best not to think of anything as being a ‘short cut’ through the fishless cycle though; bottled bacteria may be able to speed up the fishless cycle in some instances but you will still need to understand the process and test the water to ensure that the nitrogen cycle is working properly before you add any fish to your tank.

How do I do a fishless cycle to get my bacteria going so that when I put my fish in the tank the bacteria are ready for them?

There are a few methods, all of which you’ll find recommended in varying degrees on the internet. For all of them you will need to invest in your own test kit. A liquid test kit is preferable as these are, in our experience, a little more accurate. You will need to be able to test for:

  • Ammonia
  • Nitrite
  • Nitrate
  • pH (a measure of acidity)
  • GH (general hardness of the water)
  • KH (carbonate hardness of the water)

Whichever method you choose for your fishless cycle, you need to get your tank and equipment set up first. You don’t need to have the décor perfected as you can fiddle about with this while the bacteria develop. But you will need to set up your tank, fill it with water, set up the filter and media and start it running.

Method One – the fish food method

With this method, fish food is added to the tank to produce ammonia as it starts to rot. The easiest way to think of it is to imagine you are feeding invisible fish. Every day you simply add the amount of food you would give the fish if they were there. As the food rots it produces ammonia. This starts the bacteria growing.

Alternatively, you can place some fish food in a pop sock, or a cut off end of a pair of tights (note – if you’re planning on using your girlfriend’s/wife’s/mum’s it’s generally best to get permission first as we can’t be held responsible for the actions of ladies who find their favourite hosiery full of fish food and floating in a tank!). As the food rots the tights will keep it in one place and make it easier to remove when you no longer need it.

Method Two – the prawn

Pop a prawn into a pop sock, or a cut off bit of tights and place it in the tank in the flow from the filter. As the prawn rots it will produce ammonia in a similar way to the fish food method above.

Method Three – adding household ammonia

Household ammonia can be purchased from DIY stores and sometimes pharmacies. Household ammonia is usually diluted to a certain percentage; this will be stated on the bottle. You can use our calculator to work out how much ammonia you need to add to your tank.

You will need a small syringe to measure out how much ammonia you need to add, you can get these from pharmacies, or have a look on eBay or online stores for them.

Note – never, repeat never, be tempted to sniff the contents of a bottle of ammonia out of curiosity. This almost inevitably leads to a feeling that your nostrils are melting and your eyeballs are on fire.

Method Four

There are various kits available which are intended to help you do your fishless cycle in readiness for your fish. We recommend Waterlife’s Biomature (please note, we have no affiliation to Waterlife or any other company, we recommend this because it’s a good product) which just needs to be added as per the instructions. It contains a source of ammonia and tells you how to work out how much you need to add.

How do I know if I’ve got enough ammonia in my tank to get my bacteria going and start my fishless cycle?

You need to make sure that you are growing enough bacteria to be able to cope with the change from you adding a source of ammonia, to your fish adding the ammonia. It’s generally accepted that 2-3ppm (parts per million) is the best level of ammonia to aim for while cycling your tank. This means that when your cycle is complete you have a nice big colony of bacteria that won’t be overwhelmed by the amount of waste your fish throw at them.

This is where your test kit comes in. When you test the water for ammonia the colour of the sample in the test tube will change. The colour will match (more or less) to one of the colours on the test kit chart. This will tell you what level of ammonia is in your tank water.

Using household ammonia or the Waterlife kit is the easiest way of knowing exactly how much to add. Using fishfood or prawns can be a bit trial and error as you may need to up the amount of food you are using if the ammonia readings are not getting high enough. If you are using household ammonia you can use our calculator to work out how much you need to add.

If you are using fishfood you can use our calculator (don’t be confused by the title, it’s multi talented!) to help you work out roughly how much to add. Simply enter the percent of protein in the food (this will be shown on the pack) and the volume of water in the tank. Then enter 10 in the ‘daily food amount in grams’ box and click ‘calculate’. The result for ‘Per day the protein will break down to:’ is given in milligrams per litre (m/g) for ammonia which is equivalent to parts per million. Once you know how much ammonia the protein in 10g will give you, you can adjust the amount up or down until the calculator gives you about 2-3m/g or 2-3ppm.

Doing your fishless cycle

This is a good time to get children involved if your tank is in a family home, or for a child’s pet fish. Helping children to understand the needs of the fish is a great way to get them enthusiastic about the aquarium. If they can get involved in the cycling process with you that will really help them feel part of the ‘team’, not to mention the benefits of helping them understand a little chemistry and biology without them realising. Children are the fishkeepers, aquarists, conservationists of the future and can really benefit from learning about looking after living creatures and developing in confidence as they take care of other little lives. You can print off our cycling diary for your children to fill in (and for you to fill in!), just click on the link below:

Fishless Cycle Diary Template

Before adding an ammonia source

  • Set up your tank with water, décor and filter (also heater if you are going to keep tropical fish)
  • Work out how much water you have in your tank. You can use our calculator to help you, simply add the measurements requested by the calculator
  • Test the water for PH, GH, KH and nitrate before you start. The first three can change during the cycle and affect it so you need to know where you started from. Nitrate is usually present in tap water to some degree so you need to know what you started with in order to see if it is rising as your cycle progresses. Make sure you write down the results as you may need to refer back to them.  To test PH you should take a sample of tap water and let it stand for 24 hours, this will give you a more accurate PH reading
  • If you’re using household ammonia, work out how much you need to add by using our calculator

Next …

  • Add your ammonia source and test to make sure you’ve added enough (if you are using fishfood/a prawn you will need to wait a few days for ammonia to show, so test after a couple of days)
  • Test again the following day and add more ammonia if required to bring the reading back up to 2ppm. You may not need to add any more for a couple of days, but keep checking daily and write down your test results every time
  • Keep adding ammonia whenever the level goes below 2ppm to bring the level back up to 2ppm, you can use our calculator to work out how much extra to add
  • If you are using fishfood/a prawn you may need add more if ammonia levels start to drop
  • Test daily, keep a note of your results, add ammonia when needed
  • Try and do your tests and add ammonia at the same time each day
  • Check PH, GH and KH every few days – a drop in PH or KH can indicate a problem with the cycle, see our trouble shooting section for how to deal with this

After about a week …

  • Ammonia levels should be dropping. If they’re not, don’t worry, every cycle is individual and it may take a few more days.
  • Test for nitrite. You may need to wait a little longer than a week, but keep testing every other day and watch for nitrite to show. You will probably get readings for both ammonia and nitrite, ammonia levels will start to decrease as the cycle develops and nitrite rises
  • Keep adding ammonia to keep the levels up to 2ppm, this should start to drop more rapidly as more and more bacteria develop and process it faster
  • Nitrite will start to rise
  • Test every other day and keep a note of your results
  • Keep checking PH, GH and KH every few days

After about 3 weeks …

  • Ammonia levels should be dropping rapidly now
  • Keep testing and adding ammonia though as you still need to keep your ammonia bacteria happy while the nitrite bacteria develop
  • Start testing for nitrate, it may not have started rising yet as it usually takes between four and six weeks for this to happen, but it’s always worth checking just in case
  • Keep checking PH, GH and KH every few days

After about four to six weeks …

  • Nitrate will start to rise. Nitrate is the end product of the nitrogen cycle so when you see nitrate rising you know that your cycle is well on its way and nearly complete
  • Ammonia and nitrite should be showing as a zero reading within twenty-four hours of adding ammonia to 2ppm
  • Keep checking PH, GH and KH every few days

Finally …

  • Ammonia and nitrite are being processed to zero within twenty-four hours of adding ammonia to 2ppm
  • Nitrate has risen (sometimes dramatically

Woo hoo!  My fishless cycle is done! Can I finally get some fish now, please?

Almost … before you add any fish you need to do a large water change to dilute the nitrate that’s built up. Don’t worry; this won’t harm your carefully tended bacteria (but do remember to use a water conditioner).

  • Do two water changes of 50% each: do one of 50%, refill the tank, and do another of 50% and refill. This will reduce the nitrate levels to similar levels to the tap water
  • If you’ve added cold water, make sure the temperature is right for the intended residents
  • Double check everything else is as it should be; PH, GH, KH, that you know what food etc. you need to get when you’re buying your fish
  • Go shopping!

If you’re not adding fish as soon as your tank is ready, you need to keep on adding ammonia every day to keep the bacteria healthy and happy.

Trouble shooting

If your fishless cycle seems to have got to a certain point and stopped you need to check the following:

  • PH – a low PH can cause problems with the fishless cycle. Below PH 6 the bacteria can become much less active and may even stop processing toxins and die off.
  • KH – low carbonate hardness can also cause problems. Carbonates are used up during the nitrification process so if KH falls too far there won’t be enough carbonates in the water for the nitrification process to work.
  • Nitrate levels – nitrate over 100ppm has been identified on some internet forums as causing a problem with the fishless cycle for various reasons.

But fear not, there simple remedies to get things back on track. To raise carbonate hardness and PH at the same time you simply need to add some bicarbonate of soda. You can buy this in the cookery section of most supermarkets, but don’t confuse it with baking powder as these are not the same. To work out much to add you will need to know what your KH is now, and what you want it to be, if you aim to keep it above 5 that should keep your cycle ticking over nicely. Once you know what your current reading is you can use our calculator to work out how much to add.

Once you’ve added the bicarbonate of soda test the pH. Adding bicarbonate of soda will raise pH but it’s not a straightforward correlation so you will need to check every time you alter the KH.

In a soft water set up KH can drop quite quickly so it’s a good idea to get used to checking it and raising it during your cycle so that you will be able to keep it stable when you have fish living in your tank. If KH drops too far when you have fish in the tank it will cause a ‘pH crash’ where the acidity levels in the water rise dramatically as the pH ‘crashes’ downwards. This can wipe out a whole tank before you realise what’s happening.

To deal with nitrate above 100ppm do two consecutive 50% water changes (don’t forget the dechlorinator) and make sure you match the temperature of the new water to that of the old before adding to the tank. You can use boiled water from the kettle to bring it up to temperature if needed. Do one 50% change, refill, then do another 50% change. This means you’ve removed a lot of nitrate without risking upsetting the bacterial colony by adding 100% new water.

Tips for speeding up your fishless cycle

If you have another tank with a mature filter you can take some of the media from that and put it in your new filter. This will kick start your bacterial colony and help it develop faster. Equally, if you have a friend with a mature tank you could ask if they’d be willing to donate you a bit of mature media. Some fish shops will also give you some mature media so it’s always worth asking at your local shop as well.

Heating the water will encourage the bacteria to grow faster so adding an aquarium heater and keeping the temperature at about 80 degrees Fahrenheit, or 26 degrees Celsius will speed the cycle up a little.

The Instant Cycle

This applies if you’re moving your set up to a new (and usually bigger) tank. All you need to do is take all the media from the old filter and put it in the new one. You have the same amount of fish, producing the same amount of waste therefore the amount of bacteria needed to process the waste is the same as you already have in your filter so hey presto, instant cycle. You will need to monitor ammonia etc. for a few days to make sure everything is continuing as it should after the move.

If you don’t want to use your old filter media, maybe the new one takes different shape sponges for example, you can simply set up your old filter on the new tank and run it alongside the new filter for a few weeks. Every week you can remove a bit of the old media (you can cut sponges up with scissors if you’re not keeping them) as the bacteria will start to colonise the new filter media. Keep testing the water regularly to make sure everything is stable and that you’re not removing too much of the old filter media too soon.

Further help and advice

For further help and advice please see our recommended forums section. All the forums we mention have experienced fishkeepers who are well versed in the nitrogen cycle and fishless cycling and who will be happy to give you personal advice based on your individual situation.

Glossary of Terms

If you’re not sure what some of the terminology refers to you’ll more than likely find your answer in one of the glossaries linked to below.  If not, visit one of our recommended forums and someone will be sure to help you out:

Glossary of terms – from
Glossary of terms – from, use the glossary drop down on the right of the profile search

Author: INJAF

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