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How to Do a Fishless Cycle

The easy and cruelty-free way to fill your filter with the necessary bacteria before you add fish.

Sadly many new fish die in new set ups due to ammonia and nitrite poisoning. Getting the tank ready (a process that takes about 4 weeks) by adding ammonia - your pretend fish - to ensure the bacteria in the filter is in sufficient quantities to deal with fish poop.

The Nitrogen Cycle is a fixed biological process that can not be sped up, only the addition of pre-seeded filter media from a mature tank will help.


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    Set up your tank, but do not add any fish.
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    Leave the tank running for 24 hours to ensure that the equipment is working as it should be and there are no leaks.
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    Calculate the volume of ammonia required to raise the level in the tank to 2-4ppm. The level you choose is personal preference. Using 2ppm is not usually problematic during a cycle and is probably more common. Some choose to use a slightly higher level so they can house more stock when complete, but high ammonia can cause a cycle to stall and play catch up sometimes, so keepers must be aware. Use pure ammonia (sometimes called "janitorial ammonia". Some household-cleaning ammonia has dangerous chemicals and scents added. It's become harder to find since 9/11 (bomb making) and at a push fish food can be used - a large handful in a stocking - but this is not as accurate to read/dose. There are a few calculators on the web (for example, on the Fish keeping website or PFK) which require you to know the volume of your tank and the daily ppm of ammonia you have chosen in the tank - very helpful.
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    Add the required volume of ammonia to the tank.
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    Test the water every 24 hours and wait to see the ammonia reading drop. At this point the filter contains bacteria that break down ammonia into nitrite, so the cycle has commenced. Keeping a cycling diary is recommended so you can plot the changes/patterns emerging.
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    Add more ammonia to raise the level in the tank back up to the level you chose to start with. Keep testing for ammonia every 24 hours and now test for nitrite too.
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    Wait until the nitrite level starts to drop, (they will be sky high for a few weeks) continuing to add ammonia to feed the bacteria in the tank. Nitrite is often the most annoying reading as once it drops it will linger at low levels until sometimes just a few days before the end of the full cycle.
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    When you have 0ppm of ammonia, 0 ppm of nitrite, and a nitrate reading, your tank is ready for fish.
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    Continue adding ammonia until you add your fish. This will keep the bacteria from dying. Then, do a large water change (70-90%) to get your nitrate under 40ppm. Once the fish are added, continue to test the water for both ammonia and nitrite to ensure that no spikes occur.
    • Be aware that tank pH can swing around quite a bit during these 4 weeks. Not unusual. Worth keeping an eye on it. If pH falls to 6 or below, this can cause a cycle to stall but there are easy ways to rectify this. A good fish-keeping forum will be able to advise in detail on a bespoke, tank by tank basis.


  • The type of ammonia you use matters. The ammonia should consist of just water and ammonia (ammonium hydroxide is identical to aqueous ammonia). It should have no scents, colors, or anything else. If it foams when you shake it, it is no good.
  • Only add a few fish at a time. This will let your filter adapt to the extra bioload.
  • Strip-type tests are unreliable. Use liquid tests for accurate results.
  • Consider a planted tank. Plants consume nitrate, the end product of the nitrogen cycle. High levels of nitrate can be toxic for fish, so plants are a good way to keep your tank healthy. Plants can only remove so much nitrogen, though, so nitrate levels should be tested fairly often and water changes will still need to be performed.
  • Carbon filters need to be replaced often and are, therefore, not recommended to be used alone as you will have to start cycling anew each time the filter media is replaced. If you want to use carbon, have longer-lasting sponge, floss, or ceramic filter media as well to host your bacteria colony.
  • Betta fish and guppies are a great choice!


  • Always make sure the levels of ammonia and nitrite are 0 before you add any fish. Nitrate levels should be below 40.

Things You'll Need

  • A fish tank
  • A filter (see tips)
  • A bottle of Household Ammonia (see tips)
  • Test kits for ammonia and nitrite minimum (nitrate is also beneficial).
  • Bucket and siphon for water changes
  • After cycled, fish

Article Info

Categories: Aquariums