“Cycling” A New Freshwater Aquarium

One of the most common questions asked at fish stores is “I just bought an aquarium, and now what?”.

For most people just starting an aquarium, the “…now what?” means “what fish can I put in my aquarium today?”.  The correct answer is “none today”, and is generally not what the customer wants to hear.   Following this advice will result in a much more satisfactory result, however.

This process of starting the aquarium is often referred to as “cycling”, which is the introduction into an aquarium of various types of bacteria which utilize the ammonia and nitrite (both toxic to fish) produced by fish waste.  This process is accomplished by reducing ammonia and nitrite to nitrate, which is not toxic to fish.  This process (cycling) takes an average of 30 days after the introduction of the fish.  It can take as little as 21 days, or as long as 60 days without any apparent reason for the differences.

There are live bacterial cultures on the market, which can help “cycle” an aquarium faster.  These products do work when the bacterial cultures are viable, but fish should still be added very slowly.

The following steps are recommendations on how to start a new aquarium while minimizing the hassles and problems:

1. Decide on the size and type of aquarium you want to have.

2. Decide on the type of filtration you’re going to use.  You can choose from under-gravel filters, hang-on-the-back filters, canister filters, overflow filters, or some combinations of these types of filtration.  Ask your pet store associate to help you decide which type of filtration is most appropriate for your aquarium.

3. Set up the aquarium with all of the equipment and add the water.  This will include rinsing the gravel, installing the filtration, and setting the heater to the appropriate temperature.  Goldfish and other cold-water fish do well at room temperature, while tropical fish need temperatures around 78-80°F.

4. Run the aquarium for 2-4 days before adding any fish.

5. Use starter fish to begin the “cycling” process.  Some excellent starter fish include danios, black tetras, and white clouds.  Some other recommendations could include platies, other tetras, or some barbs.

Do not use too many fish during this “cycling” process.  Invariably beginners ask if it’s all right to start with angelfish, catfish, plecostomus, or other inappropriate fish.  Resist the temptation to do this, and you will save yourself a lot of grief and disappointment during the first few months of operation.

6. When you get your starter fish home, float the bag in the aquarium for 15-20 minutes to equalize the water temperature.  This is very important, as fish are very sensitive to temperature changes.  After equalizing the temperature, you can add about ¼ cup of water to the bag every 15 minutes for 1-2 hours.  The fish can then be released into the aquarium.

If at all possible, net the fish out of the bag into the aquarium, rather than dumping the water from the bag into your tank.

7. Be very cautious when feeding your fish, especially until the “cycling” is complete.  Overfeeding is the most common mistake made with new aquariums.  A fish’s stomach is probably about the size of its eye, so feed very sparingly.

Your fish should eat everything you feed them within 3 minutes.  If not, you probably fed too much.  Just reduce the amount the next time you feed.  Fish only need to be fed once a day.

8. After about 14 days, you can bring in a water sample to be tested for ammonia and nitrite.  This will tell whether the tank has begun “cycling”.  It can also tell you when it’s safe to start adding more fish.

It is not a good idea to introduce additional fish once the aquarium has started to “cycle”.  The ammonia and nitrite levels will typically rise to toxic levels during this process.  Because you started with hardy fish, they will often survive these toxic levels.  Because the increase happens so slowly, they are able to adapt with no adverse effects.

To introduce new fish during this process can be very stressful to the new fish, since they haven’t had time to slowly acclimate to the elevated levels of ammonia and nitrite.  Unfortunately, they often don’t survive this trauma.

9. Once the test on your aquarium water determines that your tank is safe, you can begin adding additional fish.  Your pet store associate can help you determine which fish are compatible in terms of size and temperament for your aquarium.  Add new fish in stages.  It’s not a good idea to add a lot of new fish all at one time.

10. Do not be disturbed if your aquarium becomes cloudy of hazy during the first several months of operation.  This is normal, and usually disappears naturally after 2-3 months.

11. Routine tank maintenance should begin after the “cycling” process has been successful.  Water changes of 20-25% should be performed every 2-4 weeks.   Fish do not respond well to significant chemical changes in their water.  They do much better with small water changes done more frequently, than with massive water changes done infrequently.

Adding water to the aquarium to replace water that has evaporated is not a water change.  Again, be very sensitive to water temperature when doing water changes.

Denny Rogers


By Denny | November 22, 2010 @ 10:35 pm | 3 comments

3 comments on ““Cycling” A New Freshwater Aquarium

  1. This page is helpful.

    I am wondering about the lights on the tank cover.
    We will have one goldfish living in a 10 gallon tank. The tank is on an interior wall and does not ever get direct sunlight. Is it ok to just turn the light on when the children need it to see in the tank? thank you.

  2. Sorry for the delay – my son just reminded me that I need to check my “comments”. By all means turn on the lights so you can see the fish. The only thing to watch out for is having the light on too much can cause excessive algae growth. Frequent water changes and or algae eaters (snails, octocinclus, bush nosed plecos are my recommendations) would help to control excessive algae growth. Denny

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