How To Cycle A New Aquarium With/Without Hardy Fish?

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Cycling a new fish tank is a very important step to make it ready for stoking fish. Neglecting to get your aquarium cycled before stocking will result in the fatality of your newfound aquatic buddies, frustration and quite possibly the end of your now very short lived interest in aquariums… In this article, I am going to give you a brief understanding of how this works including everything you need to do to have a successful aquarium cycled and to keep your little buddies from going belly up.

What is nitrogen cycle in an aquarium?

Nitrogen cycle in aquariums. Source

Fish waste and urine, uneaten fish food, and dead plant matter (if you have live plants) all turn into ammonia. This is toxic to your fish and will kill them. Beneficial bacteria (Nitrosomonas) in the aquarium feeds on the Ammonia and turns it into nitrites. This is also toxic to your fish and will kill them just like the Ammonia. Another beneficial bacteria (Nitrospira) in the aquarium feeds on the Nitrites and turns them into nitrates. This is relatively harmless to your fish. This process never stops happening in your aquarium and is called the nitrogen cycle.

In a cycled aquarium, helpful nitrifying bacteria colonize the surface of substrate and filter materials, mostly biological filter media. They help to covert harmful ammonia and nitrites to neutral nitrates and protect the living creatures from these nitrogenous toxins. In a new setup aquarium, the brand new filtration doesn't have any of these helpful nitrifying bacteria living in it yet that means nothing protects the fish from ammonia and nitrites and they will die.

Two methods for cycling an aquarium

Cycling an aquarium after all is the process of helpful bacteria enrichment in the aquarium and filter to make them ready for fish waste treatment. By using an ammonia source and first nitrifying bacteria culture, you can easily cycle your aquarium at home by yourself.

There are two popular methods for cycling a new aquarium, with hardy fish or fishless cycle. The first method uses hardy fish to produce ammonia for feeding nitrifying bacteria while the later uses commercial ammonia preparations or fish foods to do this work. Each method has its own advantages and disadvantages.

  • The cycling method with hardy fish allows for adding several fish very soon (24 hours) after the setup but it may sacrifice some or all of the fish until the tank gets cycled.
  • The fishless cycle requires a long period (1 month) before you can add the first fish to the aquarium but it ensures all ideal conditions of the tank for fish to thrive in.

If you can't wait for at least a month until having several fish swimming in your aquarium, choose the cycling method using hardy fish. If you don't want to risk any of your aquatic pets, choose the fishless cycle.

Method 1 - Cycle an aquarium with hardy fish

After setting up your new fish tank, pick out (or already have in mind) one or two hardy fish to put in your new aquarium. Go to the store and buy them and then put them in your fish tank. Make sure you acclimate them properly.

Right now, your fish are producing ammonia through waste. Luckily, you purchased hardy species that are able to endure some of this ammonia for the time being. Because you only have several fish in the tank, there is not too much ammonia production taking place to kill the fish. Do not over feeding your fish at this stage because uneaten food in the water produces toxic ammonia as well.

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After adding several fish to your tank, do daily tests for ammonia, nitrite and nitrate levels. I strongly recommend the API Freshwater Master test kit with pH, high range pH, ammonia, nitrite, nitrate tests, which are necessary for aquarium setup and maintenance.


You will see ammonia spike first, that is normal. Then ammonia levels will drop and nitrite levels rise (as Nitrosomonas bacteria that convert ammonia to nitrite becomes established). Then the nitrite level drops as another population of bacteria (Nitrospira) flourishes and converts the nitrite to nitrate. At the point where both ammonia and nitrite levels are zero, you have successfully cycled your new aquarium and the process of stocking the aquarium can begin.

If it gets too out of control, you can do "mini" aquarium water changes but please, absolutely no more than 10% of the water and 3 times a week maximum. Remember, we need that ammonia in there for the bacteria to feed on. Keep an eye on your fish, if they are swimming sideways or acting twitchy, that is not good and you should do a small water change.

You are now free to SLOWLY add more fish to your aquarium. I advise that you still refrain from adding a bunch at the same time. Every time you add a new animal to the tank, the ammonia output increases and the bacteria must reproduce to meet the increased demand and return to equilibrium. In a mature tank, this process can be completed in hours. In a newly-cycled tank, it will take longer, possibly days or weeks.

Hopefully you did this without any fish deaths on your hands and if you followed all directions this should be the case. If you did endure a loss, don't feel bad; sometimes it happens even when you do everything correctly.

Method 2 - Cycling without fish (fishless cycle)

The main purpose of this method is to propagate helpful nitrifying bacteria on your aquarium filter materials and substrate as much as well, before stocking the first fish. You will need ammonia and a starting bacteria culture to kick start the process.

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Ready-to-use ammonia and bacteria culture are commercially available for cycling new aquariums. I strongly recommend the DrTim's Aquatics Ammonium Chloride (ammonia source) and API Quick Start Aquarium Nitrifying Bacteria.

You can use a small amount of water, substrate, and filter media from an old aquarium (from your other aquariums or your friends) to add bacteria to the new setup tank. You can also use fish food to produce ammonia instead of buying an ammonia solution; however, it may need some time for the food to decay.

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After adding ammonia and a bacteria culture to your tank, add nothing during a month but test ammonia, nitrite and nitrate levels (preferably daily). You can use separate kits for testing ammonia, nitrite, and nitrate levels; however, I strongly recommend the API Master test kit with 800 tests for pH, ammonia, nitrite, and nitrate testing. Please note that you not only need these tests for the setup but also will use them for scheduled water quality confirmation or occasionally measurements.


Ammonia will spike first. Do not be surprised if it exceeds several parts per million. Since the tank has no livestock, this is not a problem and is a normal part of the process. When ammonia starts dropping, nitrite will climb. When it starts dropping, nitrate will climb. Do not be surprised if your tank does not perfectly follow this pattern. When ammonia and nitrite hit zero and nitrates are present, your tank is cycled. Now you can add some fish to your tank.

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1 thought on “How To Cycle A New Aquarium With/Without Hardy Fish?”

  1. I am using an under gravel filter with 3 powerheads in a 54 gallon corner tank. Set it up 3 days ago. Put in 6 zebra Danio today. Water conditioned with sea chem prime. No plants yet. No HOB filter. Will my tank cycle ok or am I in trouble?


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