Goldfish Water Cycling

Cycling Your Fish Tank 

Please include attribution to with this graphic.


Share this Image On Your Site

The health and happiness of our finned friends are critical to their potential lifespan. Goldfish are capable of living for decades. However, most will likely die before they even reach five years old. This is mainly due to a lack of sufficient space for swimming, breathing, eating, growing, and playing, as well as neglectful owners and poor water quality. There are ways to help your fish live long and happy lives. One of which is to provide good quality water for them to live in.

What qualifies the tank water as “good quality”?

For aquarium water to be considered “good quality” it needs to be first treated with a water conditioner before it is even added to the tank. The water conditioner is used to make tap water safe for fish to live in. Straight tap water is full of chlorine and heavy metals. While the levels of heavy metals and chlorine may not be high enough to harm us as humans, it does create a toxic environment for fish. If you have treated your tap water and have added it to your aquarium, then you need to know about a process called “tank cycling” (which we will discuss in a minute).

Other ways to be sure your water is “good quality” is to provide a good filtration system. Filters with Mechanical medias, Biological medias, and Chemical medias are designed to remove unwanted materials from the tank water. Mechanical medias take care of physical floating debris, Biological medias breakdown dissolved physical floating debris and uses it to help build up the beneficial bacteria, and finally, the Chemical medias remove any chemical materials from the water including copper, chlorine, and other unwanted drugs or chemicals. Without a filtration system your water will be dirty, cloudy, discolored and dangerous for your fish to live in.

Performing a 30% water change every two weeks is another way you can help keep your tank water in good quality. These water changes help eliminate nitrate and slow the production of unwanted algae. When replacing the 30% old tank water with 30% new tank water, be sure to treat it with a water conditioner before adding it to the main tank. Any amount of untreated tap water will cause your fish to go into shock, become stressed and ill. Be sure to test your tank water’s temperature. A goldfish aquarium should always fall between 62ºF and 75ºF. Water temperature recommendations vary with each species of fish. Do your research!

You will also need to test your tank water for levels of ammonia and nitrite. These two chemicals are toxic to fish (and most animals). However, if your tank is properly cycled, then your ammonia and nitrite levels should be zero. Finally, testing your water for pH levels and understanding what they mean will help you keep your tank water in “good quality.”

What is tank cycling?

The term “tank cycling” is another way to refer to the nitrogen cycle. It is the process by which bacteria is purposefully built up on or in your aquarium filter cartridge. Beneficial bacteria may also build up on the gravel, décor, and plants (artificial and live). The bacteria may take a few days to start, but, once it does then it is time to feed it. Cycling methods will be discussed in detail below.

How do you feed bacteria? Well, your fish will actually do this part for you! Some of the bacteria that are building up in your tank will feed on the ammonia that is excreted from your fish’s waste. As your fish produce waste, the ammonia-eating beneficial bacteria will consume the ammonia from it.  As they consume the ammonia they also release it but it is no longer ammonia. Instead, it has been transformed into nitrite. Nitrite is also very harmful to your fish. Luckily, if your tank has been properly cycled, then there will be nitrite-eating beneficial bacteria developed as well. These bacteria will then consume the nitrite. The nitrite is then transformed into a chemical known as nitrate. Nitrate will contribute to the production of algae as well as cause your fish to become stressed. If you keep up with bi-weekly water changes, then your fish should not be affected by the nitrate.

Do not be afraid of the word bacteria. In regards to your aquarium and the overall health of the inhabitants, a beneficial bacterium (which is what we are talking about) is a good thing and it is actually essential for keeping your fish alive.

Why is tank cycling necessary?

There are chemicals and other unwanted materials that are harmful to your fish. One of the best ways to eliminate them is with beneficial bacteria. Ammonia and Nitrite are detrimental to your fish. The worst part is that it comes from them! Their waste contains ammonia which needs to be eliminated for your fish to remain healthy. Consider how you would feel if you had to live in a place surrounded by the chemicals from your own waste. Tank cycling creates good bacteria which are in charge of eliminating these harmful chemicals and keep your fish alive! That is why cycling your tank and building up the beneficial bacteria is so important.

What is the proper method for cycling a tank?

There are a few different ways to cycle your tank. You can cycle with fish or without fish. We will discuss each method in detail.

How to cycle your tank with fish:

  • Completely set up your aquarium (do not add fish at this time)

a.  Lay the gravel you bought on the aquarium floor (3 to 4 inches thick is recommended)
If you chose an undergravel filter, install it before you add the gravel (hence the name undergravel filter) (Do not plug it in)

  • Place the plants and other still objects in the tank as you want them set up. If you chose live plants to be sure to bury them well because most fish species are diggers and will uproot the plants.
  • Install any air pumps or air stones (Do not plug it in)
  • Install the filtration system (if you chose an undergravel filter then this should be fitted before the gravel) (Do not plug it in)
  • If you purchased a heater then this should also be installed (Do not plug it in)
  • Finally, it is time to add water. The water should have already been treated with a water conditioner and should be included in the tank slowly to avoid splashing or damage to anything inside.

Time to add fish!

  • Do not just add any fish! There are particular species that are known as being good cycling fish and those should be the fish you add at this time. Other species of fish may not be able to handle the high level of toxic materials at this time and should only be added when cycling is completed and water is safe. The fish that are recommended for cycling are hardy fish and can handle these beginning conditions well. A few examples of cycling fish include:

Cherry or Tiger Barbs
Most minnows or guppies
White Clouds
X-ray Tetras

  • For other cycling fish options consult a knowledgeable staff member at your local pet store.
  • You should only use a few cycling fish to start. Too many fish in these conditions would create even more harm



  • During the cycling process it is important that you do not feed your fish too often or too much
  • Most cycling fish can be fed once a day, every other day
  • Avoid feeding too much as this will cause uneaten food to sit at the bottom of the aquarium. The uneaten food will then produce more toxins in the water and create an even more harmful environment for the fish. You also do not want the fish to produce too much waste and raise the toxin levels faster than the bacteria can develop.

Water changes

  • Perform a small water change every few days during the cycling process.
  • 10% to 20% of the tank water should be removed and replaced with new treated tap water.
  • Careful not to add the new water at a different temperature than the old tank water
  • More frequent water changes may be wise if you notice your fish are in ammonia stress

Test the water for toxins daily!

  • By monitoring the toxin levels in the tank water daily, you can avoid the levels spiking up too high.
  • The toxin levels will drop as the beneficial bacteria that we mentioned earlier begins to develop and mature.
  • Ammonia levels should be 0.5mg/L, and nitrite levels should be 1mg/L during the cycling process. If they read lower than these measurements, then you are in good shape! Ideally, you want these levels to be zero or as close to zero as possible.
  • When the toxin levels read as close to zero as possible then the cycling process is completed, the good bacteria are mature and are taking care of business!

Time to add more fish!

  • Do so, gradually. For example, if you planned on inhabiting 10 small fish in total and you only added two or three for cycling then you should add two to three more small fish at a time. Allow those fish to adjust to their new home and then repeat until all of your fish are introduced to the aquarium.
  • If you were to introduce all of your remaining fish into the tank at one time (after cycling) then you could cause an ammonia or nitrite spike thus destroying all the hard work you and your cycling fish have done over the past few weeks.
  • Only add new fish when the levels are low

How to cycle your tank without fish:

Completely setup your aquarium:

  • This process should follow the same instructions as previously listed (i.e. placing your gravel, plants, electrical devices, adding water slowly, etc.)


  • Yes, you will be “feeding” during this process. The amount of food that you would normally feed your fish (about a pinch or two of fish flakes) should be added to the aquarium water
  • No, there aren’t any fish living in the tank yet to eat the food and you don’t want that in this case
  • When you are providing the food and it goes uneaten then it will rest on the bottom of the tank, becoming moldy and will thus release the toxins (especially ammonia) that your fish would have
  • If you had fish in the tank you wouldn’t want the food to go uneaten and produce more toxins because then the ammonia and other toxin levels would be far too high and would be dangerous for the fish

Testing the water:

  • After a few days of moldy food producing toxins, it is time to test the toxin levels
  • Ammonia should read about 3ppm (parts per million)
  • You can bring a sample of your tank water to the local pet store to be tested (if they allow that) or you can purchase your own kit and perform the test yourself
  • If the ammonia level is not at least 3 parts per million then you will need to add another pinch or two of fish flakes to the tank and wait another few days
  • Once the ammonia levels are at a steady 3ppm it is important that you continue to keep them here. The good bacteria are developing and consuming the ammonia and by keeping the ammonia levels at 3ppm you are feeding the bacteria and helping it grow.
  • If the ammonia levels decrease, just add more fish flakes to the water
  • Allow this process to continue for about a week

Testing for nitrites:

  • Ammonia-eating bacteria are likely developed and mature at this point. Therefore, the ammonia levels should be decreasing.
  • However, the nitrite levels are likely increasing because the ammonia-eating bacteria are now releasing nitrites.
  • Test the water for nitrite levels.
  • Once the nitrite levels begin decreasing, you know the nitrite-eating bacteria have matured and are doing their job.

Final test during cycling:

  • Once the ammonia levels and nitrite levels have decreased (because the bacteria are consuming them) it is now time to test for nitrates
  • Nitrates develop only when cycling is almost completed
  • Clean the bottom of your aquarium with a gravel vacuum or siphon. This will help remove any remaining food that has settled on the gravel.
  • You can begin adding your fish slowly once the ammonia and nitrite levels are undetectable. Follow the previously mentioned method for adding fish to the aquarium.

Are there other ways to cycle a tank?

You can speed up the bacteria development process if you have an existing tank. Only use the already active tank filter from another aquarium for a bacteria boost in a new tank. You could also speed things up by adding some gravel from an active tank, or even by adding live plants to the new tank. Live plants are carriers of beneficial bacteria. They also benefit from the ammonia levels because they can use it to assist them in protein synthesis. Talk to a knowledgeable staff member at your local pet store for help choosing live plants.

Before adding an active filter or gravel from an active tank to a new tank, be sure that the active tank is free of parasites and harmful algae. If the existing aquarium has had sick fish in the past, then avoid using anything from it in the new aquarium.

What impurities am I testing the tank water for?

Ammonia stage: The ammonia stage is when you are attempting to raise the ammonia levels just enough to feed the growing beneficial bacteria, but, not so high that it harms the cycling fish. As the beneficial ammonia-eating bacteria develop, they will begin consuming the ammonia and thus, ammonia levels will decrease.

Nitrite Stage: The nitrite stage is after the ammonia stage. The beneficial ammonia-eating bacteria that have been consuming ammonia are now releasing nitrites. The nitrite levels will rise until the nitrite-eating bacteria mature and begin consuming it. As they consume the nitrites, the nitrite levels will decrease.

Nitrate Stage: This is the final stage of cycling a tank. Nitrates are produced after nitrite levels decrease. Nitrates are not harmful to your fish as long as the there are not high levels. Nitrate levels should be controlled by water changes performed regularly.

What should I use to test my aquarium water?

You can take a sample of your tank water to the local pet store to be tested (some stores even do it for free). However, not all pet stores offer this service, or you may even want to perform the tests yourself. I recommend using the API Freshwater Master Test Kit. This kit is fully equipped with the necessary equipment needed to test your aquarium water for pH level, ammonia, nitrite, nitrate and high range pH. These tests have been proven to be scientifically accurate and provide the consumer with professional results. You should be testing your water regularly during the cycling process and then at least weekly after cycling has completed.

How do I perform these tests with this product?

Each brand will provide specific step-by-step instructions on how to use the equipment as well as how to test, how to read the results and other useful information. It is important that you follow these specific instructions in order to accurately test the water.

How much does it cost?

The API Freshwater Master Test Kit can cost anywhere between $20.00 and $30.00 depending on where you purchase it. Some mom and pop pet stores may charge even more.

More information to know about your tank water:

Ammonia Stress:

  • Fish are resting at the bottom of the aquarium
  • Lethargic
  • Inflammation around the gills, eyes, or their caudal fin area
  • Struggling to breathe at the water’s surface

Treat ammonia stress by performing a water change for often and in slightly greater quantities. You can also consider trying an ammonia neutralizer that can be purchased at most local pet stores.

Goldfish Aquariums:

If you intend on having a goldfish aquarium, then you should use a hardy goldfish breed to cycle their tank. For example, the Comet or the Common goldfish. Do not use goldfish in cycling tanks unless the tank is going only to contain goldfish after cycling.