Articles Setting up an aquarium Water quality

Aquarium nitrogen cycle & cycling a fish tank

February 16, 2013
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After setting up your aquarium it can very tempting to immediately head to the pet store and buy a bunch of fish. Unfortunately, if you do this, you’ll often find your new aquarium inhabitants upside down within a week. This is something that happens to many new aquarists.

It can be very demotivating to have all your fish die almost instantly, but fortunately these unnecessary fish deaths can easily be prevented by cycling your aquarium.

All new aquariums go through a stage where they are a toxic, extremely dangerous environment for any live animal before turning into a healthy system. During the ‘toxic’ stage, beneficial bacteria colonies that can turn wastes into less harmful substances establish in the filter and gravel, but unfortunately, they take a while to do so. This is called the nitrogen cycle.

To understand when your aquarium is ready to add fish and what you should do if you’ve already introduced them, it’s important to know what the nitrogen cycle is first. Keep reading for more info on the nitrogen cycle or skip to how to cycle an aquarium.

The nitrogen cycle

The nitrogen cycle and the growth of beneficial bacteria are triggered by the presence of ammonia (NH3), which is released into the aquarium water as soon as fish feces, uneaten fish foods and rotten plants start breaking down. Ammonia is very toxic to all aquarium inhabitants, and thus, ammonia levels should be kept at 0 ppm (parts per million) at all times if there are fish in the tank. This is where the cycling process comes in.

Water test results from a cycling tank after ~1,5 weeks. Left-right: ammonia, nitrate, nitrite.

  • If you introduce fish in an uncycled aquarium and test the water regularly, you will notice the ammonia levels rising around the third or fourth day. Nitrifying bacteria species, often called ‘beneficial bacteria’ by aquarists, will now start to do their job.
  • As the bacteria settle in the filter and gravel of your tank and start eating the ammonia, you will notice nitrite (NO2) levels rising. Nitrite, which is a by-product of the ammonia-eating bacteria, is also very toxic to fish and can kill/permanently damage them even at very low levels. Luckily, a second colony of bacteria should now start forming.
  • Nitrites will be converted to much less toxic nitrates by this second colony. Nitrates are the final product of the cycle, and will stay present in the aquarium unless a water change is done. Plants also use nitrates as nutrients, but they are no replacement for regular tank maintenance.
  • As soon as nitrates are present in the aquarium and ammonia and nitrite levels have dropped to 0 ppm, the aquarium is ‘cycled’. This means that after a water change is done to reduce the nitrate levels to <30, fish can safely be introduced. You may now run out to the store (just don’t introduce too many fish at once).

How to cycle an aquarium

There are several ways to cycle an aquarium; most involve introducing ammonia in the tank to trigger the growth of beneficial bacteria colonies. I recommend getting a water testing kit before starting the cycle – this way you can monitor the water values and determine in which stage of the cycle the aquarium is. A liquid tester like the API Freshwater Master Test Kit is much more accurate than test strips and cheaper in the long run as it can last you many years.

Cycling with fish: not a good idea

The oldest and most common way to cycle an aquarium is cycling with fish. This means some cheap fish are put in the tank so the ammonia they excrete starts the cycle. It is often recommended by pet stores because it means extra profit for them when fish die, but cycling an aquarium this way actually takes a long time, requires a lot of water changes to keep the fish alive and usually permanently damages or kills the fish used. Luckily, there are plenty of cruelty-free and easier ways to cycle an aquarium.

Cycling with food

As mentioned earlier, uneaten fish food is broken down into ammonia, which is exactly what we want in this case. A large, uncooked prawn or fish food is added to the tank and left there to decay until the tank is cycled, and that’s it.

This method is not entirely risk free, because rotting food often attracts unwanted bacteria and turns the tank water very dirty. A big (90-100%) water change should always be done before introducing fish to a tank cycled this way.


Cycling with ammonia

Pure household ammonia is sold in garden stores and some supermarkets as a cleaning product. Ammonia is also sold specifically for aquarium use in smaller containers, such as this one. Add about a drop of ammonia per gallon of tank water. After ammonia levels drop to zero, repeat this and wait until ammonia and nitrites are both at zero.

Your aquarium is now ready for fish. A large water change should be done to lower the nitrates to <30 ppm before introducing them, and if you’re not ready to introduce them yet, be sure to keep adding ammonia to prevent your beneficial bacteria from starving.

Seeding the filter

Using cycled filter media that already has the right beneficial bacteria colonies in it can quick-start the cycle in your aquarium. This way you can add fish instantly, but you do run the risk of introducing any diseases or pests from the ‘donor’ tank into the new one.

If you don’t own a cycled aquarium, you can ask friends who own an established tank or pet/aquarium stores – they might be able to supply you with some seeding material. Use the media as quickly as possible – it should be transported in some water from the tank it was in and it should never sit for more than ~1 hour before it is put in the new tank’s main filter or a separate one. If you need to let the used media sit for a longer time, put an air stone into the bucket to provide the beneficial bacteria with oxygen and flowing water and prevent them from dying.

Bottled bacteria

Filter bacteria can be bought from pet-/aquarium stores in a bottle. It is recommended to always check the expiration date on these products as they can usually only be stored for about a year, and to keep them in a cool, dark place like the fridge (not the freezer!).

Don’t assume you can add fish right away: these products are best used as a ‘kickstart’ for your cycle so the process doesn’t take as long. They do work, though, especially when combined with a high quality water conditioner like Seachem Prime to remove traces of ammonia and nitrite.

Cycling your aquarium before adding any fish is one of the most important steps to achieve a healthy little ecosystem, and it’s one you should never skip. The ammonia and nitrite spikes that result from failing to properly cycle an aquarium cause fish death and weeks/months of trouble, constant water changes, having to buy expensive products, and  can completely ruin the hobby for you.

If you have any more questions about cyling an aquarium or if you have something to add to this article, be sure to leave a comment below. Happy fishkeeping!

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  • Reply Jessica June 22, 2016 at 7:51 pm

    I have a 150 gallon tank that I’m trying to prepare to set up. TRYING. I plan on getting Orandas.
    I called a local pet store (Petsmart) to find out about cycling and some information about drift wood curing- the lady I spoke with said that if I bought water conditioner I would not need to cycle. Is this the bottled bacteria method of cycling? I know big brand pet stores aren’t the best to get trusty info from but there are no privately owned stores here. Are there any major tips and downfalls to the goldfish varieties? Also what are your thoughts on driftwood vs foe coral for gold fish varieties.

    • Reply Mari June 26, 2016 at 9:23 pm

      Hello! Apologies for the delayed reply, although I assume you’re still in the middle of the cycling process and haven’t bought fish yet. Great to hear you’re doing your research and have gotten a nice big tank. I would definitely try to understock it so the fish have all the room they need to grow. I would personally probably only go for about four goldfish in a 150 gallon.

      The lady in the pet store was very incorrect. Water conditioner is needed to remove harmful chemicals such as chlorine from tap water and some types are indeed effective in removing ammonia as well. However, this is just temporary and not sustainable like actually cycling the tank. You’ll need a product like Quick Start if you want to kickstart the cycle, but even then I definitely wouldn’t recommend adding fish right away.

      As for goldfish varieties, I used to have an oranda but it unfortunately passed away due to swim bladder issues. They are lovely fish but if you get young ones it can be a little difficult to see whether they will end up with a monstrously huge wen. I personally prefer the ones that are a little less extreme! It’s also easier to buy adult fish because there is less chance of hidden issues that come up once the fish ages, but they are definitely a little more expensive. If there’s any possibility to do so, I would avoid buying goldfish from big brand pet stores – the stock is often of bad quality and it’s so discouraging to immediately have to deal with diseases etc. Try finding a local goldfish breeder/importer or a site like Dandy Oranda’s/Raingarden Goldfish, they ship goldfish throughout the US.

      I love driftwood in goldfish tanks, but definitely make sure it has no sharp edges because these fish are mega clumsy. Spider wood, for example may not be ideal, something like Mopani wood is probably a better idea.

      If you haven’t had a look yet be sure to check out the goldfish category here on Aquariadise, there are a few articles in there that might be helpful in your research.

      Good luck! 🙂

  • Reply Sam barker April 1, 2016 at 9:17 am

    Hi there,
    Would appreciate some advice,
    Have a tank that I set up bought some safe start as it said would cycle my tank immediatly making it safe for fish. Left it 2 days before putting in a couple of guppy. I then bought a test kit it’s been 10 days since fish went in and my test results are ammonia 0.2 (these have dropped from 0.5) nitrite 0.25 ( have also dropped from 0.5) nitrate has stayed at 5 from when I started testing.
    I read online not to do a water change while the tank is cycling with bottled bacteria as it can interrupt the cycle but unsure if these levels are safe for the fish?
    The test kit indicates danger at a higher level but dont want the fish to be uncomfortable, do these results show it’s cycling at all?
    Any advice would be greatly appreciated.
    Kind regards

    • Reply Mari April 3, 2016 at 12:07 pm


      Oh dear, products like safe start do help kick start your cycle but if it says it will instantly cycle your tank that’s not really true. It seems your tank is cycling, but there may not be enough beneficial bacteria to deal with the guppy waste yet (this can also be caused or made worse by a tank that’s too small – at least 15 gallons for guppies!)

      I would definitely start doing water changes. It does disrupt the cycling process, but that’s the annoying part about fish-in cycling: to prevent the fish from getting hurt, you have to constantly disrupt the process which can cause it to take much longer than cycling without fish. I don’t have a fish-in cycling guide, but there is one here (just scroll down to the fish-in cycling part). You’re not in the worst possible situation right now because the tank is already partially cycled, but it could take a while for everything to become stable.

      I hope that helps! Just be sure to cycle your tank beforehand next time, products like safe start can help speed up the process but it’s not a good idea to completely depend on them. 🙂

      • Reply Sam barker April 3, 2016 at 3:16 pm

        Hi ok
        I guess it’s a learning curve and I won’t be so niave in future Lol.
        Thanks so much for the advice I will start water changes ASAP, what % and how often would you recommended?

        • Reply Mari April 3, 2016 at 3:56 pm

          It’s definitely a learning curve for every aquarist, such a pity there’s so much bad info out there. I’d suggest sticking with the instructions from the article I linked to – 25% change daily until everything is stable.
          Good luck! 🙂

    • Reply Paul Smith April 18, 2016 at 10:51 am

      Hi, been cycling a new setup, 25 gallon low tech planted. I got Ammonia and nitrite readings around day 7,1 ppm and 0.75 ppm respectively. Ammonia dropped to 0 within 2 days and nitrite after 5. Meanwhile I was picking up nitrates at low levels, 0.1-0.3. I did a partial change, 30%, and swished out the filter media. The day after, my ammonia and nitrate readings came back up to 0.5 and 0.4 ppm, and nitrate raised a little also. since then ammonia is back to zero but nitrate remains at 0,25 and nitrates are barely present, < 0.1.

      my questions are –

      1. were my day 7 readings high enough to indicate a cycle?
      2. Did i wash off too many bacteria swishing out, causing the tank to re-cycle?
      3. I did not add fish food or ammonia (oversight) until day 7, but many of the plants were melting and i had a few tiny snails (~10mm). would this have been enough load to cycle?
      4. Since day 7 I have daily dosed with fish food (flake), hoping to build up my colony. Is this sensible?
      5. Finally, I have 2 big, large leaved java fern, 4 large anubias, 4 small pointy leaved crypts and 3 rotala rotundifolia. would these plants likely cause my nitrates to remain so low?
      Thanks so much for any advice on this.

      • Reply Mari April 18, 2016 at 11:47 am


        It seems things were going well – is there a reason you swished out the filter media? That could indeed wash off bacteria and isn’t necessary during the cycling process. Water changes aren’t necessary either unless you’re cycling with fish.

        Your day 7 readings did indicate cycling, but they were very low and the cycle would probably not have been stable enough to sustain any fish. The melting of your plants and the snail poop likely caused the small ammonia, nitrite and nitrate readings. What you’re doing now, doing with fish food, is a much better plan. You could also switch to dosing with ammonia as described in the article as a less “gross” method. Once you do that you should start to see higher ammonia, nitrite and nitrate readings. The plants will likely suck up quite a bit of nitrate but I think you’ll still get higher readings than before.

        I hope that helps! If I was unclear anywhere feel free to ask any more questions.

  • Reply Ginger January 29, 2016 at 5:43 am

    If I seed a tank, I can add fish the same day? Asking because I’d like to get my betta into his own tank ASAP so I can medicate him again. He’s in a 15g with some cories rn.

    • Reply Mari January 29, 2016 at 7:54 am

      Hi! Sorry to hear you’re having problems with your betta. Seeding a tank usually doesn’t instant cycle it unless you move an entire cycled filter to it. I would allow at least a day or two to see to which degree the seeding has been successful and how the water values react (using your liquid water test kit). May I ask what’s wrong with your betta?

  • Reply Shannon signal December 9, 2015 at 1:45 pm

    Hey I just upgraded to a 10 gallon from my 1 gallon. Unfortunately before I could move my betta he died. I had him for months and just found out his home was too small but it was too late not to mention I cleanse everything without soap which may have killed him. Well now I have a 10 gallon. It’s set up with water, sand, decorations and a hand made rock bridge. I put in aquasafe tap water conditioner and two days later added my snail that was with my betta. When can I add fish and what more is needed for my tank because 1 gallon to 10 is a big jump and I’m lost. 10 gallon was a gift so I didn’t do any research.

    • Reply Mari December 9, 2015 at 8:46 pm

      Hi! Glad to hear you’re doing some research, but sorry about your betta 🙁

      The article you commented on contains all the information you need, but I understand it can all be a bit difficult to figure out if you’re not sure what to do yet! You have to get a filter and heater for the 10 gallon if you don’t already have these. They need to be turned on. You also need a liquid drop test kit like the freshwater master test kit, which seems to be most popular in the US. You use the test kit to keep a close eye on the water values and determine when the tank is ready for fish: it’s never the same so I can’t tell you an exact time frame, but it will usually be at least around three weeks. The cycling game then begins! How to cycle is described in the article super detailed, so that should help you. Don’t add any fish until that process has fully finished, or the tank won’t be safe.

      I hope that helps, but if you have any more questions feel free to ask. I listed a few articles below that might be helpful to you. Just keep reading, reading, reading and you’ll figure it out in no time. Be sure to let me know how things go, good luck 🙂

      How to care for a betta
      Betta tank inspiration
      What aquarium maintenance should you be doing?
      Equipment checklist
      8 easiest aquarium plants

  • Reply Cliff October 21, 2015 at 1:47 pm

    Hi I put two newts in a 5.5 gallon fish tank . I didn’t cycle it sorry didnt know about cycling in till a few weeks ago. So i started to cycle my tank. It has been 12 days . I noticed my small newt always trying to get out buy scratching the glass not climbing on land? Didn’t think much of it. Then 2 days ago my big one starts scratching the glass and climbing on land/rocks. I have 4 1/2″ of water i drop it to 3 1/2″ and fill it back up to 4 1/2″ with reptisafe . Every other day for 12 days. I bought test stripes Walmart only hade stripes no kit. It said ammonia .25 nitrate 0 nitrite 10.0. So i did a half water change then another half change couple hours later.know it saying ammonia .25 nitrate 0 nitrite between 1.0 and 3.0 should i do more? Sorry killed one betta frome cleaning everything no soap with toothbrush. He lasted a year i didn’t now i was killing him. Some of his fines fell off not all put him in shallow dish.scales rased up like bine cone died in 3 days poor guy. I think he liked me little did he know i was killing.

    • Reply Mari October 21, 2015 at 9:55 pm

      There’s a couple things wrong with your setup right now unfortunately. I’m not a newt expert at all, but a 5.5 gallon tank is much too small for them. I would recommend bringing the newts back to the store and then reading up on articles like these before starting with fish/amphibian keeping again! You really need to know how things work beforehand, as just winging it is not a great idea at all when live animals are involved.
      When you’re ready to set up an aquarium again, you can use the 5.5 for small inverts like dwarf shrimp or dwarf crayfish or a single betta fish. Nothing bigger, unfortunately! 5.5 gallons is really, really small.
      Good luck!

      • Reply Cliff October 21, 2015 at 11:34 pm

        Well that not to good for my newts . I got them of eBay i cant just send them back . Hope i can just do daily 25% water changes and everything works out.

  • Reply Darcy October 15, 2015 at 1:17 am

    Hi there
    Iv recently set up my new 60 gallon tank with a used Aqua one 1000 canister filter. New substrate and rocks.
    My question is since I already have about 10 fish I am planning on transferring because of the filter can I add them all straight away or gradually? If so how many and how frequent?

    • Reply Mari October 15, 2015 at 7:40 pm

      Hello! What a coincidence, I’m cycling a 60 gallon with a used canister filter as well. 🙂
      I don’t know what types of fish you keep, but transferring gradually would be great if the situation allows it! You could split it into three transferring moments with a week in between. As for the frequency, to figure that out you should just keep a close eye on the water levels: if the ammonia levels seem to remain at 0 where they should be, you can add the next “batch” of fish after a week or so. If there are fluctuations, wait until they are over to add more fish.
      I hope that helps a bit!

  • Reply Mimi January 21, 2015 at 6:42 pm

    Hmm, I have a cycled tank already, but would like to move my tank critters (2 guppies and a CPO) to a larger tank (so from 20L to 60L). I added from the brand Tetra, their EasyBalance and AquaSafe to the tap water in the new tank. Do I still have to cycle the tank first? And if I do, can I just put the used filter from the old tank into the new tank in order to get the good bacteria in the new one?

    • Reply Mari January 21, 2015 at 7:46 pm

      You can put the filter from the old tank into the new tank to move the bacteria so you don’t have to cycle. You’ll have to add another filter as well, though, because one meant for a 20L tank won’t be enough for a 60L. Be sure to keep a super close eye on your cycle and do tests with a liquid test kit every day or every other day, because you will likely experience some cycle bumps!
      Good luck 🙂

      • Reply Mimi January 22, 2015 at 5:49 pm

        Thanks for the response!
        I’ve put in the old filter cartridge into the new tank and will wait a few days so hopefully everything will work out!
        (oh, and the new tank came with it’s own filter, so not to worry 🙂 )

        • Reply Mari January 24, 2015 at 12:15 pm

          Great! Just be sure to test and retest before you move the critters over. Good luck 🙂

  • Reply Alvin Yao October 5, 2014 at 5:15 am

    The leaves on my anubias have started to die, and they are rotting in my tank. I didn’t have time to cut them off, so when I tested the water in my aquarium, it had 0.5ppm amonia, and 0ppm nitrite. Since I have no fish in the tank, and already have beneficial bacteria in the tank, would this count as cycling the tank?

    • Reply Mari October 5, 2014 at 1:48 pm

      If you already have beneficial bacteria in your tank I would assume it is already cycled. If not, then this would indeed likely produce some beneficial bacteria. I wouldn’t count it as “cycling”, though, as 0,5ppm ammonia isn’t that much and you wouldn’t have a stable nitrogen cycle.

      • Reply Alvin Yao November 3, 2014 at 12:13 pm

        I used bacteria in a bottle which is supposed to make cycling alot faster. I added it on the first day I finished setting up my tank. The water was cloudy from shrimp soil. The media browned in the first couple of days. I only added fish a week later, and when the water was crystal clear. Ammonia was 0.5ppm and Nitrite was 0ppm. It continued to be like that for many days untill one day ammonia became 0. there was never a nitrite reading, ever. I think this is wierd. Is the combination of the rotting plant and shrimp soil
        for the first wek when I wasn’t checking for ammonia or nitrite that provided ammonia to feed the bacteria and keep it alive, and allow the bacteria to settle onto my filter mefrom starving?

        • Reply Mari November 4, 2014 at 1:34 pm

          Interesting that you didn’t get a nitrite reading, although maybe depending on the kind of tests you used something went wrong. If there was “rotting” material in your tank I think that definitely played a role along with the bacteria in a bottle, although I can’t tell you anything for sure! For the tank to have cycled there must have been nitrite, but the test may not have picked it up.

  • Reply Morgan Lefford June 16, 2014 at 5:28 am

    I was thinking about getting a betta and I’ve been researching. That being said, does a tank smaller than five gallons need to be cycled? And if not, why? Thanks!

    • Reply Mari June 16, 2014 at 1:03 pm

      Tanks under 5 gallons do cycle, they just don’t hold the cycle as well: if there’s a single piece of food decomposing in a 10 gallon, it probably won’t be a problem. There is a lot of water to dilute the ammonia that’s released and the larger amount of good bacteria can easily convert it. In a 2,5 gallon, the ammonia concentration will be much higher, there likely aren’t enough good bacteria and nitrite/ammonia spikes will occur. The whole cycle will be off, which can be fatal to your fish.

      Tanks < 5 gallons should be cycled as well, but ideally they shouldn't be used at all because it's so hard to maintain a stable cycle. I'd really recommend you go for at least a 5 gal (if you're worried about the costs, you can get them used really cheap), just to prevent any problems in the future. Experienced fishkeepers can usually keep bettas in slightly smaller tanks, but I personally don’t see why you’d want to. It’s more work, there’s a much higher risk of accidental fish death, it doesn’t look as nice and a 5 gal really doesn’t take up much space at all.

      Hope that clears things up for you! If you’re still interested in getting a betta, this article contains tons of info about proper betta setups. Good luck 🙂

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