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  • Writer's pictureStewart Kessel

Setting up an Aquarium - Fishless Tank Cycle

Updated: 2 days ago

A step-by-step guide to fishless tank cycling

A fishless tank cycle is a technique used in newly set-up aquariums to establish the bacterial colonies required to remove toxic waste created by livestock. You must complete the process before the addition of your chosen fish and other livestock, hence the term fishless.

The bacterial colonies are required as part of the nitrogen cycle to help mature your aquarium filter. Once established Nitrosomonas species bacteria will convert excreted toxic ammonia to nitrite and Nitrobacter species bacteria will, in turn, convert the nitrite to less harmful nitrate. Read our previous article on the nitrogen cycle for more detailed background information and understanding of the importance of this process.

What’s Required?

Source of Ammonia: There are various methods to incorporate ammonia. The most reliable technique is to add liquid ammonia direct, rather than, for example, adding aquarium food and allowing it to rot. Household ammonia is suitable as long as it is unscented and with no other additives. Alternatively, there are several aquarium specific sources of ammonia.

Water Test Kit: A good-quality aquarium water test kit is necessary to chart to the progress of the tank cycle. As a minimum, you will require a test kit for ammonia (NH3), nitrite (NO2) and nitrate (NO3). However, test kits for pH and calcium hardness KH are also advisable. Choice of the test kit is very much dependant on personal preference, but a good quality liquid test kit is generally more reliable than test strips.

Water Conditioner (De-chlorinator): Household tap water typically contains chlorine and or chloramine. Both are toxic to your fish and other livestock, including the beneficial filter bacteria. Therefore, before adding water to your aquarium, or during water changes, you must precondition the water with a reputable water conditioner (de-chlorinator).

Step-By-Step Process

There are four basic steps to completing a fishless cycle:

Step 1 Ammonia dosing

Step 2 Maintain ammonia and watch nitrite

Step 3 Large water change to reduce nitrate

Step 4 Stock your Aquarium

To further understand the process, these main steps are described in more detail below:

Diagram showing fishless tank cycle - relevant ammonia, nitrite and nitrate levels through the process

Step 1 Ammonia dosing

1.1 Set-up your new aquarium and equipment, e.g. filter and heater. Fill with water remembering to treat the water with a water conditioner (de-chlorinator).

1.2: Turn on the equipment and check for correct operation, e.g. temperature and water flow from the filter. Note there is no need to turn on the lights during this process unless you have introduced live plants.

1.3: [Optional] seed your aquarium filter either with some media from an already matured filter or add a commercially available filter aid.

1.4: Dose your newly set-up aquarium with ammonia to a level of 4ppm. Either follow the instruction on the supplied source of aquarium ammonia or use an online calculator to determine the required addition to achieve an ammonia level of 4ppm.

1.5: Using the test kit, determine the ammonia content (t=0) to validate your addition. Note if the level of ammonia is higher than 4ppm do a particle water change to reduce the amount down to 4ppm. If the quantity is below 4ppm, then add additional ammonia to achieve the desired level.

1.6: Repeat step 1.4 & 1.5 every day, testing for ammonia level and adjusting to 4ppm. This process is to start the cycle, i.e. stimulate initial growth of Nitrosomonas species bacteria colony and keep the bacteria alive by feeding them ammonia, their source of nutrition, at a suitable concentration. Note it is very likely that in the first few days the ammonia level will remain unchanged and no additions will be required.

Step 2 Maintain ammonia and watch Nitrite

2.1: After approximately a week, when you start to see the ammonia levels drop, begin to test for nitrite. As the first part of the nitrogen cycle ammonia is converted into nitrite, so when you detect nitrite, this is a sign that the cycle has started.

2.2: Continue testing for ammonia every day. Whenever it drops below 4ppm add enough ammonia to bring the level back up to 4ppm. Also, test for nitrite every other day. You should see the nitrite rise and then start falling after a few weeks.

2.3: Start testing for nitrate after a few weeks when the nitrite levels start to fall. Nitrate is the last part of the process where the Nitrobacter species bacteria convert the nitrite to nitrate. When the test kit starts showing a fall in the nitrites, you should see a rise in the nitrates.

2.4: Continue dosing the ammonia during the cycling process as in step 2.2 until it is evident that the cycle has established. At this time, the dosed ammonia and resultant nitrite will both fall to zero within a 12 to 24-hour period, and the nitrate level will significantly increase.

2.5: Continue to run with additional ammonia for 1 to 2 days after step 2.4 to confirm that the cycling is complete.

Step 3 Large water change to reduce Nitrate

3.1: After confirming that the cycling is complete, dosing of ammonia can stop, and a water change is required to reduce the nitrate level.

3.2: Several significant water changes may be needed to reduce the nitrate to a safe level (typically less than 40ppm).

Step 4 Stock your Aquarium

4.1: As soon as you have completed step 3, you can follow this by the addition of your chosen fish and other livestock, whose waste will maintain the cycle. Please see advice about stocking levels.

Note: You must stock the aquarium as soon as the cycling is complete; otherwise, there will be insufficient nitrogen nutritional source for the filter bacteria, and the colony will start to die back. A good tip, if you are not ready to stock, is to continue with step 2.5, but keep an eye on the Nitrate level as if this goes too high as this can also stall the cycling process.


Why 4ppm?

The value is set at 4ppm to simulate a typically well-stocked aquarium. Cycling at a lower amount and then stocking at the same level will put a too high a bio-load on the filter and could cause issues with excess ammonia or nitrite in your aquarium water.

If you only plan to have a low stock level or have a nano aquarium (30L or less) then cycling at a lower ammonia dosing level 2-3ppm may be advisable as this will be quicker to establish. Please ensure you read advice about stocking levels.

How to do I calculate the required dosage of ammonia?

If you are using liquid ammonia as recommended, then you can calculate the theoretical amount you need to add based on the w/w % strength (should be noted on the bottle), the volume of water in the aquarium and the desired ammonia level. To save the maths, you can use an online calculator such as the one on our web site

If you are using another source of ammonia such as a proprietary aquarium product, then follow the instructions on the container. If you are using a natural source of ammonia such as fish food, then, unfortunately, this is not practical to calculate and must be established by trial and error.

Can you add too much ammonia?

Yes, excess ammonia can cause the cycle to stall. Ammonia levels in excess of approx. 5 ppm can prevent the cycle from completing due to an excessively high pH and inhibiting the Nitrobacter species bacteria from growth. At too high a concentration, greater than approx. 10ppm, the free ammonia will also inhibit the ammonia utilising Nitrosomonas species bacteria.

Should I add plants?

There is no reason why you cannot add plants, the plants themselves are not affected by the cycling and in fact, use the nitrogen sources as a source of nutrition. The plants will also generate oxygen as part of the photosynthesis process, which will help ensure good dissolved oxygen levels, as required by the bacteria and other livestock once added after the cycling. See our separate article on the benefits of a planted aquarium on our web site

Do I need to turn on the aquarium lights?

Unless you have added live plants, see above, then there is no need to have the aquarium lights on during the cycling process. Doing this will help reduce the likelihood of nuisance algae. The beneficial bacteria are photophobic and therefore, not only do they not need light, but are shielded through purposely designed filters.

Is the process different for a coldwater set-up?

The simplistic answer is no, the process and step-by-step procedure are the same. However, due to the lower water temperature, the process will typically take longer for the beneficial bacteria colony to get established.

Is the process different for a heavily planted aquarium?

No, the process and step-by-step procedure are the same, but there are some additional factors to keep in mind. Many aquaria soil-substrates leach ammonia with time; this is not an issue with a fishless cycle but will affect the measured ammonia level. Similarly, the plants will use nitrogen sources as nutrients, again affecting measured readings. Typically, aquascapers use a low final fish stock density and therefore, it is possible to use a lower ammonia dosing level during the cycling process.

How long does it take to complete the cycle?

There is no precise answer to this. Many factors can affect how long each stage of the cycle take. However, typically a cycle may take 4 to 6 weeks to complete. Seeding the cycle with mature filter media can shorten the process, but in reality, it takes as long as it takes.

Does temperature matter?

Yes, the temperature is a factor. The bacteria will grow more quickly when the water is warmer, with drops in temperature slowing the process. Some experiments suggest a rise of 4°C increased ammonia oxidation by 50% and nitrite oxidation by 12%. Dropping the temperature by just 1°C reduced ammonia oxidation by 30%. Although there may be a temptation to run the cycle at an elevated temperature to reduce the time take to complete the process, the bacteria will, in turn, take time to re-acclimatise to the lower temperature required in the final system once stocked.

Potential Issues

Low KH & or pH

When pH and KH drop, the nitrifying bacteria slow down and stop multiplying. A sudden drop in KH and hence pH is typically associated with consumption of the buffering minerals in the aquarium water. This situation is more prevalent in soft water areas or with the use of RO water with insufficient remineralisation.

Excessively High NH3

Excess ammonia can cause the cycle to stall. Ammonia at greater than approx. 5 ppm can prevent the cycle from completing due to a high pH and inhibiting the growth of the Nitrobacter species bacteria, required to convert the free ammonia. At concentrations greater than approx. 10ppm, the free ammonia will also inhibit the ammonia utilising Nitrosomonas species bacteria.

High NO2

Even at relatively low levels, this will inhibit the ammonia utilising Nitrosomonas species bacteria, and when coupled with high ammonia levels as described above this can stall the cycle, not giving the Nitrobacter species bacteria a chance to establish.

High NO3

High levels of nitrate can inhibit both the ammonia and nitrite utilising bacteria as a result of low pH due to conversion to nitrous acid.

Author: Stewart Kessel CChem, MRSC

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