How to Cycle Your New Fish Tank

Updated on August 8, 2019
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I am a scientist, a day gecko breeder, and a tree frog keeper.

Cycling a new fish tank is essential for keeping the inhabitants alive
Cycling a new fish tank is essential for keeping the inhabitants alive | Source

Why Do Fish Die in a New Tank?

When you set up a new fish tank, establishing the biological filter, known as 'cycling' the aquarium, is the most important thing you can do to ensure the fish you put into the tank don’t die within days. Failure to do it will almost certainly result in the 'new tank syndrome,' which is just as scary as it sounds. You lovingly set up the aquarium, fill it with crystal clear water, carefully select your fish, slowly place them in the tank making sure that you give them time to acclimate, and everything looks great for a couple of days. But soon all your new pets are gulping frantically at the surface, then going belly up and dying.

The dead fish fell victim to ammonia a by-product of their metabolism. All fish produce ammonia, and it is lethal to them. In lakes, ponds, and rivers, the volume of water is so big, that the ammonia produced by fish is diluted to undetectable levels. However, in a home aquarium, the volume of water is much more limited, and ammonia levels quickly rise and kill the fish. Although the water in a new tank appears very clean, it is in fact, toxic.

What Is "Cycling" a New Fish Tank?

Cycling refers to establishing populations of special bacteria in your aquarium and filter which will carry out the nitrogen cycle. These nitrogen-fixing bacteria actually like ammonia, and they will take it from the water and turn it into relatively harmless nitrate. This is in fact a two-step process. First one type of bacteria produce nitrite, which is still harmful to fish, then a second type will take the nitrite and metabolize it to nitrate.

You don’t actually need to add the nitrogen bacteria to your aquarium, they are everywhere and will naturally seed the biological filtration sponge in the filter. The problem is that when you first set up your new fish tank, the bacteria are present in very small numbers. There is not enough of them to cope with the waste of your fish. As soon as ammonia levels begin rising, the ammonia fixing bacteria start to multiply, but it takes time for them to fully populate the filter sponge. After a certain period of time, ammonia levels start falling, however, nitrite levels will start rising, this stimulates the second type of nitrogen-fixing bacteria to multiply and start using up the nitrite. After a while nitrite levels will start falling. A properly cycled tank has undetectable levels of ammonia and nitrite.

Nitrogen cycle in an aquarium
Nitrogen cycle in an aquarium | Source
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The danio is a small hardy fish that can be used to cycle a new aquariumThe discus is very sensitive to water conditions and should only be added to an established tank.In the alkaline water needed by Malawi cichlids nitrite is as toxic as ammoniaDwarf cichlids like this Apistogramma are beautiful but sensitive.
The danio is a small hardy fish that can be used to cycle a new aquarium
The danio is a small hardy fish that can be used to cycle a new aquarium | Source
The discus is very sensitive to water conditions and should only be added to an established tank.
The discus is very sensitive to water conditions and should only be added to an established tank. | Source
In the alkaline water needed by Malawi cichlids nitrite is as toxic as ammonia
In the alkaline water needed by Malawi cichlids nitrite is as toxic as ammonia | Source
Dwarf cichlids like this Apistogramma are beautiful but sensitive.
Dwarf cichlids like this Apistogramma are beautiful but sensitive. | Source

How to Establish a Biological Filtration

The nitrogen-fixing bacteria are present everywhere, but to multiply in the filter, to levels sufficient to handle the ammonia produced by a fully stocked tank they need a source of ammonia and nitrite. The traditional way for cycling a tank involved adding a few small feeder goldfish. However, this always seemed silly to me, unless you are specifically setting up a cold water goldish aquarium. I mean what do you do with the goldfish once they’ve done their job? Actually, I prefer not to know.

An acceptable method to me seems to be to add a few (or one, depending on the size of the tank), of the fish species that you intend to keep in your tank. Obviously, choose the hardiest of the fish. For a general community tank zebrafish (danios) make good first inhabitants, or small barbs. The presence of the fish will cause ammonia levels to rise, but since the tank will be severely understocked, they will rise slowly. Regular small water changes will ensure that ammonia doesn’t rise to levels that would seriously harm the fish.

You should monitor the levels of ammonia and nitrite, either using a bought test or by taking a sample to the pet shop for testing. You should see ammonia rising slowly, and eventually it will reach a peak and start falling, while nitrite starts rising. You should watch your fish carefully for signs of ammonia stress (lethargy, gasping at the surface, inflamed gills). If the ammonia level seems to be rising too fast, increase the frequency and number of water changes.

The cycling process usually takes between 6 and 8 weeks. At the end of it, you should detect no ammonia or nitrite but raising nitrate levels. At this time, you can start adding more fish to the tank; however, only a few at a time, to allow the biological filter to adjust to the increased load. Again monitor the levels of ammonia and nitrite carefully as you add new fish.

Chemicals or Filter Boosters for the New Fish Tank

A variety of products are sold that claim to speed up the filter cycling process or even dispense with the need for it altogether. There are resins that bind ammonia removing it from the water, in essence using chemical filtration instead of biological. Most fish keepers do not use them, and the reason is obvious.

Although cycling the tank takes a while and needs careful testing, in the long term biological filtration has many advantages over chemical. The ammonia binding resins do not last forever, and eventually, they become saturated and useless, they have to be replaced regularly. Not only is that more expensive than relying on natural bacteria, but if you for some reason can’t or forget to change the resin on a fully stocked tank, the results can be disastrous.

Do not use ammonia binding resins during the cycling process, as your filter will never develop the nitrogen-fixing bacteria, because they will be starved of ammonia. However, the chemical filters could be used in emergency situations, when there is a sudden ammonia spike that threatens to kill off all your fish.

You can now buy filter boosters, which claim to provide the bacterial cultures to kickstart the cycling process. However, many people claim that they do not result in a stable biological filter, and advice aquarists not to use them.

Sometimes there is a possibility of adding chemical ammonia into the tank, rather than using fish to start the cycling process. This is not advisable, unless you can carefully calculate the final concentration of ammonia that you want. Too much can actually kill the nitrogen-fixing bacteria by raising the pH too high.

A good alternative to using fish to cycle the tank is to put in a frozen shrimp (shrimp the food, not shrimp the pets). If you are cycling a very small tank, where even small starter fish might produce harmful levels of ammonia. I do prefer to use fish, because the biological filtration adapts to the stacking levels of the tank.

Once the nitrogen cycle is well established, it should be quite stable, with all the ammonia and nitrite being taken care off by the biological filtration. It is important to take care of the filter. Occasionally, it will be necessary to clean the filter to prevent it from clogging up and affecting the flow rate. Never wash it with chlorinated water under the tap, you will kill all those bacteria that you took such care to establish. Clean the filter at the same time as doing a water change by gently squeezing it in the pail of old tank water. Also be aware that if you ever have to treat the tank with antibiotics, you will kill all the beneficial bacteria and plan accordingly.

© 2012 aa lite


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      14 months ago

      Why doesn't the article tell us what ammonia readings are acceptable during fishless cycling.

    • profile image


      4 years ago

      Just add water lettuce , it quickly absorbs ammnia, and will help eztablish the new tank


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