How to cycle a new aquarium before adding fish
Edited by Marin Hale · Every section cites its sources below it
A brand-new tank looks ready the moment it is filled, but it has none of the bacteria that make water safe for fish. Cycling is how you grow them. This guide covers what the nitrogen cycle is, how to cycle without fish, how to tell it worked, and what to do if fish are already in the tank.
- Cycling grows the bacteria that turn toxic ammonia into nitrite and then into much safer nitrate.
- Cycling without fish is the safest route for beginners, and seeding with used filter media or bottled bacteria shortens the wait.
- Zero ammonia and nitrite only count if the tank has had waste to process, so test with a full ammonia dose or look for new plant growth.
- Expect about four to eight weeks from scratch, and let the test readings decide when it is done.
- If fish are already in the tank, keep them few and hardy, feed lightly, test often, and change water when ammonia or nitrite is above 0.2 ppm.
The nitrogen cycle, simply
Fish produce waste, and that waste becomes ammonia. Ammonia is very toxic. Beneficial bacteria eat it and turn it into nitrite, which is still harmful. A second group of bacteria turns nitrite into nitrate, the least toxic of the three.
Nitrate slowly builds up. You remove it with water changes, or live plants use it to grow new leaves.
Sources for this section
[1] aquariumcoop.com: “Yellow = ammonia (which is very toxic and can burn fish gills and skin)”
[1] aquariumcoop.com: “Step 2: Beneficial bacteria #1 eats the ammonia and produces nitrites.”
[1] aquariumcoop.com: “Step 3: Beneficial bacteria #2 then eats the nitrites and produces nitrates (the least toxic nitrogen compound).”
[1] aquariumcoop.com: “You must remove the nitrates either by doing a water change or by using aquarium plants.”
Why cycling matters
A new tank has no bacteria to process waste, so ammonia can climb quickly once fish are in it. Skipping the cycle does not skip the nitrogen cycle. It just means your fish go through it with you.
Cycling grows enough beneficial bacteria, and ideally plants, to handle the waste your fish will make. A cycled tank means fewer losses and easier upkeep.
Sources for this section
[3] pond-planet.co.uk: “Brand new fish tanks do not have this good bacteria, which means fish cannot survive.”
[4] freshwatersystems.com: “If you do not cycle your aquarium, the nitrogen cycle will still occur, but your fish will likely be harmed in the process.”
[2] buceplant.com: “Cycling involves adding beneficial bacteria and live plants to a new tank to increase its biological filtration capacity.”
Fishless cycling, step by step
Fishless cycling is the best choice for most beginners because no fish are exposed to ammonia while the bacteria grow. You add an ammonia source to the empty tank and let the bacteria multiply on it.
You need a filter, a test kit that reads ammonia, nitrite and nitrate, and fish food.
- Set up the tank fully (filter, heater if you use one, substrate, decorations) and keep everything running the whole time. The bacteria settle on the filter and other surfaces.
- Seed the tank if you can. Used filter media from a healthy, established tank, or a bottle of live nitrifying bacteria, shortens the wait a lot. Only take media from a trusted source, so you do not bring in pests or disease.
- Add an ammonia source. Fish food works: as it rots it releases ammonia. Add roughly the amount you would feed if the fish were already in the tank.
- Test the ammonia. Aim for about 3 ppm. If it falls below 3 ppm, add more food. If it rises above 5 ppm, change some water.
- Test daily. Nitrite will appear and may spike quickly. Keep the ammonia supply going, because the bacteria need it.
- Keep feeding the cycle. Once nitrate shows up and nitrite begins to fall, keep adding a smaller amount of food every couple of days so the bacteria still have something to eat.
- When ammonia and nitrite both read zero and nitrate remains, run the readiness check described below before adding fish.
Sources for this section
[4] freshwatersystems.com: “While the aquarium is cycling, all bubblers, heaters, pumps, and filters should be kept running.”
[2] buceplant.com: “Only use media from a trusted source to minimize the risk of introducing unwanted pests and pathogens to your new tank.”
[4] freshwatersystems.com: “A simple and effective way to raise ammonia levels in an aquarium is by adding fish food.”
[4] freshwatersystems.com: “Check the ammonia daily for one week and add food if the ammonia drops below 3 ppm. Likewise, perform a water change if the ammonia rises above 5 ppm.”
[4] freshwatersystems.com: “Once nitrite levels begin to rise, they will spike quickly as there is nothing to regulate them yet.”
[4] freshwatersystems.com: “Once the ammonia and nitrite levels reach zero and the nitrate levels remain, the tank has finished cycling.”
How to tell the cycle is done
A zero reading for ammonia and nitrite only means something if the tank has had waste to process. An empty, unfed tank also reads zero. So the test is to give the bacteria a real job and see whether they finish it.
In a fishless tank, add a full ammonia dose. If it reads 0 ammonia, 0 nitrite and some nitrate 24 hours later, the tank is ready.
In a planted tank, new plant growth is the sign that the plants and bacteria are processing waste. In a fish-in cycle, start with a few fish, feed very lightly, and increase feeding over 4 to 6 weeks.
Once you pass the test, add fish a few at a time and keep testing. The 'Is your tank ready for fish?' section on this page has the full procedure.
Sources for this section
[1] aquariumcoop.com: “How much beneficial bacteria does the tank have, and is it enough to treat the waste produced by the fish?”
[4] freshwatersystems.com: “If ammonia levels are undetectable after 24 hours, nitrites are undetectable, and nitrates are present, the tank has completed its cycle.”
[5] aquariumcoop.com: “Once the plants (or algae) show new growth, the cycle is complete.”
[5] aquariumcoop.com: “Feed your fish very lightly at first and then gradually increase the amount over the next four to six weeks.”
[4] freshwatersystems.com: “To begin, add one or two fish so the bacteria do not become overwhelmed.”
How long it takes
Plan on four to eight weeks if you start from scratch, and be ready for it to run longer. Some tanks take a few months.
Seeding helps. With used filter media from an established tank or a bottled bacteria product, many tanks cycle in a matter of weeks. Live plants also bring some bacteria with them.
The tank tells you when it is done, not the calendar. Keep testing until the readiness check passes.
Sources for this section
[4] freshwatersystems.com: “Aquariums generally take four to eight weeks to cycle.”
[2] buceplant.com: “You can expect it to take anywhere from a few weeks to a few months to fully cycle a new tank”
[2] buceplant.com: “cycle your tank in a matter of weeks”
[1] aquariumcoop.com: “You can speed up this process by buying a bottle of live nitrifying bacteria, getting some used filter media from a friend, or growing live plants”
If you already have fish (fish-in cycling)
It is riskier, because the fish live in rising ammonia and nitrite while the bacteria catch up. It is best kept for emergencies, such as when you have fish that need a tank and no established tank is available. If that is you, these steps lower the risk.
Test every day or two. Do a partial water change whenever ammonia or nitrite reads above 0.2 ppm. Keep the fish few and hardy, and feed lightly.
Seeding the tank with used filter media or a bacteria product makes the whole process shorter and safer. Once you can feed normal amounts for a week with ammonia and nitrite at 0 ppm and some nitrate, add more fish slowly, with a wait between additions.
- Keep only a few of your hardiest fish in the tank. One small fish per 10 gallons is a common starting point.
- Feed very lightly, then increase the amount gradually over 4 to 6 weeks.
- Test ammonia, nitrite and nitrate every day or two.
- Change part of the water whenever ammonia or nitrite is above 0.2 ppm.
- Add more fish only after a week of normal feeding with ammonia and nitrite at 0 ppm and nitrate above 0, and add them slowly.
Sources for this section
[2] buceplant.com: “This method should only be considered in emergency situations where you have fish that need a tank but no established tanks are available.”
[4] freshwatersystems.com: “Fish in the tank during this process are exposed to higher-than-normal levels of ammonia, and even hardy fish can suffer as a result.”
[5] aquariumcoop.com: “When setting up your aquarium, only add a few fish at the start.”
[5] aquariumcoop.com: “Whenever you see a detectable amount of ammonia or nitrite (anything above 0.2 ppm), do a partial water change”
[5] aquariumcoop.com: “once you are able to feed your fish normal amounts of food for a week”
Common mistakes
Adding fish because the water looks clear. Clear water says nothing about ammonia or nitrite, so wait for the readiness check to pass.
Adding too many fish at once. Even in a cycled tank, a sudden jump in fish outpaces the bacteria. Add a few at a time and test between additions.
Overfeeding in a fish-in cycle. Extra food and extra waste both turn into more ammonia.
Letting the ammonia run out in a fishless cycle. The bacteria need a steady supply of it to keep growing.
Thinking a bigger filter grows more bacteria. Bacteria live on every surface in the tank, so a bigger filter only gives them more room.
Forgetting nitrate. Once the cycle is running, nitrate keeps building. Change water when it reaches 40 ppm or more.
Sources for this section
[3] pond-planet.co.uk: “you must wait to buy tropical fish until your aquarium is ready”
[4] freshwatersystems.com: “If fish are introduced to the tank too quickly or if too many fish are introduced at once, the biofilters will not be able to keep up with the amount of ammonia produced by the fish.”
[4] freshwatersystems.com: “Avoid overfeeding the fish, as this will cause them to produce more waste and lead to the additional production of ammonia via decayed food.”
[4] freshwatersystems.com: “you must maintain ammonia levels during this stage as well”
[1] aquariumcoop.com: “there is a common misconception that buying bigger or more filters will increase the amount of bacteria in your aquarium.”
[1] aquariumcoop.com: “If the nitrates reach 40 ppm or more, then you need to remove some of the dirty tank water and replace it with fresh, clean water.”
Is your tank ready for fish?
Zero ammonia and nitrite on a test only proves the tank is ready if something has been producing waste for the bacteria or plants to process. A tank that has just been sitting there with no fish, no food and no ammonia will read zero simply because there's nothing to break down. Use the route that matches your tank:
| Your tank | Ready when… |
|---|---|
| Fishless cycle (you add ammonia or fish food) | The tank turns a full dose of ammonia into nothing within a day: 24 hours after dosing, ammonia and nitrite read zero and nitrate is present. Keep adding the ammonia source until the fish go in, or the bacteria starve. |
| Planted tank (plants do the work) | The plants are visibly growing: new leaves and roots show they're using up the waste. Plant heavily and give them good light, substrate and fertilizer first. |
| You already have fish (fish-in cycle) | There's no single ready day. Start with only a few fish, feed very lightly, and increase food gradually over four to six weeks while testing often. |
Then: Whichever route you used, add fish a few at a time, feed sparingly, and test every few days for the first weeks: ammonia and nitrite should stay at 0 ppm and nitrate below 40 ppm. If ammonia or nitrite show up, stop adding fish, feed less and do a partial water change.
Sources for this procedure
freshwatersystems.com: “If ammonia levels are undetectable after 24 hours, nitrites are undetectable, and nitrates are present, the tank has completed its cycle.”
buceplant.com: “You can expect it to take anywhere from a few weeks to a few months to fully cycle a new tank”
aquariumcoop.com: “Once the plants (or algae) show new growth, the cycle is complete.”
aquariumcoop.com: “Feed your fish very lightly at first and then gradually increase the amount over the next four to six weeks.”
aquariumcoop.com: “Gradually add a few fish, feed them sparingly, and use water test strips to ensure that ammonia and nitrites are at 0 ppm and nitrates are below 40 ppm.”
This is the same procedure on every page of this site.
Next steps
Sources
- The Beginner's Guide to the Nitrogen Cycle for Aquariums (www.aquariumcoop.com)
- Cycling an Aquarium: Fishless vs. Fish-in Cycle (buceplant.com)
- How to Cycle a Fish Tank and Why it's So Important (www.pond-planet.co.uk)
- How to Cycle a Fish Tank (www.freshwatersystems.com)
- Aquarium Cycling (Aquarium Co-op) (www.aquariumcoop.com)