Cycling Your New Fresh Water Tank: Read This First!

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Dec 31, 2004
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Cycling Your First Fresh Water Tank
What is Cycling and Why is it Important?

Fish waste, and especially fish breathing, plus uneaten food and other organic matter breaking down in a tank all produce Ammonia. This can quickly become toxic to fish if it is allowed to build up to any measurable levels for any length of time. Cycling is the process of establishing the bacterial colonies in a tank that will process the toxic wastes produced so they are no longer a threat to your fish. This process takes a bit of time (5-6 weeks on average) and a basic understanding of what is going on.

Nature has provided a type of nitrifying bacteria that live in water and which consume ammonia. These bacteria also need oxygen, inorganic carbon (not from a living source) and a bit of iron to turn that ammonia into energy they can use. Unfortunately, the ammonia eating bacteria “poop” out Nitrite which is also toxic to fish. Not quite as toxic as ammonia, but close. Nature again rides to the rescue with another kind of bacteria which consumes nitrite. These also need oxygen and inorganic carbon. And what do these nitrite eaters “poop” out? Nitrate.

Fortunately, nitrate is much less toxic to fish than either ammonia or nitrite. While nature has ways to turn the nitrate into harmless nitrogen gas, they are not practical in most fresh water tanks. So the fish keeper must take steps to remove nitrate. The easiest way for most of us to do this is to change the water in our tanks regularly. This will also help to remove other things we don’t want to build up.


This cycle is referred to as the Nitrogen Cycle because ammonia, nitrite and nitrate are all based on nitrogen (N) combined with other elements. So when people talk about cycling a tank, they are talking about building up large enough colonies of the ammonia and nitrite converting bacteria to handle the ammonia load a tank produces. You will not be able to detect ammonia or nitrite in a cycled tank.

What the Bacteria Need to Thrive

You need to start with some number of bacteria in your tank which will multiply when given an ammonia source. These bacteria tend to live all over fresh water systems- in water treatment facilities, in private pipes etc. Just by filling up your tank with tap water (dechlorinating as needed), you will most likely get some of the needed bacteria. How much is hard to say. However, the more one starts with, the faster the cycling process will go.

Besides the ammonia or nitrite, the bacteria need oxygen and inorganic carbon as well as a proper temperature. While they can survive from just above freezing to about 42C (108 F), they do best between about 75F and 85F. Lastly, the ammonia converting bacteria do not function as well the lower the pH gets. If your pH drops under 7.0, they start to slow their activity markedly and under 6.5, they slow dramatically and by about 6.0 they will appear to stop completely.

To get a tank cycled in a reasonable amount of time, you need to make sure that, in addition to ammonia, the bacteria will also have:

  • Lots of oxygen by keeping the surface of your water well agitated to let oxygen in.
  • Inorganic carbon (as carbonates) by keeping your KH up. Do not let it drop below 3 dg (55 ppm).
  • A good pH level by insuring it is above 6.5. The closer to 8.0 the faster the cycle will go. We do not recommend one alter their pH if it holds fairly steady anywhere between 6.8 - 8.5.
  • An optimal temperature by having a heater set to between 75F and 85F (24C and 29C). Lower tends to slow the cycle while higher won’t make things go even faster.

If you give the bacteria all they need, they will multiply. Even more importantly, they will produce a bio-film which they use to attach themselves to surfaces in your filter and all over your tank and which protects them. In a cycled tank the nitrifying bacteria do not live free floating in the water.

How the Cycle Should Progress

The cycle starts when ammonia is added to a tank. One then waits for the ammonia consuming bacteria to reproduce to the point they will turn most of the ammonia to nitrite. As the nitrite begins to build up, the nitrite consuming bacteria will start to reproduce. Because the nitrite eaters start second and reproduce more slowly, ammonia will zero out before nitrite. Nitrate levels will appear soon after nitrite.
For many years the needed ammonia was obtained by adding a very few hardy fish to a tank and letting them create it. This was/is a gradual process and mistakes can harm or kill fish. It takes many months (6 or more) to stock a tank completely this way. In the mid 1990s some folks began doing what is called a fishless cycle. They realized one could get ammonia into a tank without using fish and that it was a
faster way to cycle a tank fully. If one makes a mistake, it just lengthens the process but it won’t harm or kill fish. Instead of taking 6 months to cycle and stock, it could now be done in 5-6 weeks, more or less.

What You Need to Cycle Your Tank

Test Kits
In order to know what is going on when you cycle your tank, you need to be testing the water. Most local fish or pet stores will test your water parameters for you, but you can get some or all of the kits if you want. We strongly advise you to own at least the kits for ammonia and nitrite, and the one for pH would also help a lot. In any case, make sure you know both the pH and the KH (carbonate hardness) of your tap water before you start. The test for nitrate is the least needed and the least accurate, but it can be useful since the final product of the cycle is nitrate.

All of the ammonia and nitrite test numbers used in this article are based on the type of tests used in the API fresh water test line. The following brand kits use similar methods and scales and can be used as well: SeaChem Multitests, Salifert Nitrite (use the nitrite ppm scale not the nitrite-nitrogen scale), any salicylate based test for ammonia that states it reads Total Ammonia (NH3 + NH4+), any nitrite test that says it reads Nitrite (NO2-) and not Nitrite-nitrogen or -n (NO2-nitrogen or –n).

Ammonia Source
Ammonia for cycling can come in two forms. The one best known is liquid household ammonia (ammonium hydroxide). It is important to find pure ammonia, that is, with no surfactants, scents or soap added. The other form is ammonium chloride. This is a dry powder which can be mixed to make any strength solution desired. However, we recommend that one find a premixed solution- it is not easy to mix at home.

Ammonia sources

Fish food or shrimp allowed to rot in a tank will produce ammonia. They will also produce some other nasty looking stuff there is no reason to have fouling a tank. This method does not allow one to have any control over how much ammonia is introduced or when. And rotting food can cause a bacterial bloom (not the bacteria we want) which turns the water very cloudy. We advise against using this to create ammonia in your tank.

Ready – Set – Cycle

Day 1
Set up your tank with the décor etc. Fill it with dechlorinated tap water and add the filter and heater. Turn everything on and wait for the tank temperature to reach the desired 75 - 85 F range. Make sure the tank runs for at least a couple of hours no matter what. You should already know the parameters of your tap water and that they are in a proper range for cycling.

Please note, the average fishless cycle should require a total of between 5 and 6 ammonia additions (Doses) as described below.

Add the initial amount of ammonia (Dose #1). This should be an amount that should produce a test kit reading of 3.0 ppm (1 ppm = 1 mg/l). We strongly advise that fish keepers new to cycling do not exceed 3 ppm of ammonia. Too much ammonia and/or dosing it too often will usually work against the process rather than helping it. Please use the dosing calculator found here Ammonia Calculator to determine the initial amount for a tank your size and for the % strength ammonia you are using. (Please read the Suggestions and Trouble Shooting section below on calculating the volume of your tank.)

Record the actual amount of ammonia you add for the initial dose as it will serve as the base amount for calculating a later maintenance addition. To be sure the ammonia has time to circulate in the water, wait about 30 minutes after adding it to test the level in your tank to confirm it is close 3 ppm. The ammonia calculator is usually more accurate than a test kit.

Now comes the hard part- you need to be patient as mostly what you will be doing is waiting and testing and waiting some more.

Days 2 and 3 Do nothing.

Day 4 Test for ammonia and nitrite.

Days 5 and 6 Do nothing.

Day 7 Test for ammonia and nitrite.

Continue testing every 3 days (Days 10, 13 etc).

If at any time after the first ammonia addition (
Dose #1) you test and ammonia is under .75 ppm and nitrite is clearly over 2 ppm, it is time to add more ammonia (Dose #2). Add the same full amount as you did the first time. Now, begin to test the ammonia and nitrite levels every other day. (You should be seeing nitrate soon if you have the kit.)

After the second ammonia addition (
Dose #2), while waiting for nitrite to rise, peak and drop, the bacteria will need a maintenance feeding (Dose #3). Give the bacteria a “snack” by adding 1/3 of the full dose when you get two consecutive every other day ammonia test readings of 0 ppm,. This “snack” (Dose #3) should be needed somewhere between days 21 and 27 of the cycle. Only a single snack dose is needed.

After the maintenance feeding (snack
Dose #3), whenever you test and ammonia is .25 ppm or lower and nitrite is clearly under 1 ppm, it is time to add another full ammonia dose (Dose #4) and then test in 24 hours.

If ammonia and nitrite both read 0 ppm, you are cycled. Do a large water change, be sure the water is the proper temperature, and add fish. The odds are this will not be the case quite this soon.

If ammonia and nitrite do not both read zero, continue to test daily. Whenever ammonia is again at .25 ppm or less and nitrite is clearly under 1 ppm, add the full amount of ammonia (
Dose #5) and test in 24 hours. Follow this pattern of testing and adding (this would be Dose #6, #7 etc.) until both tests do read 0 ppm within 24 hours. The cycle should not take much longer to be completed and even with slower tanks one should not need to go beyond Dose #6 or #7.

A major benefit of this fishless cycling method is that you can now fully stock your tank in one go. This means an average stocking level for your tank size. It certainly does not mean you can stock heavily or over stock. If for any reason you are unable to stock the tank when it is cycled, you can continue adding ammonia to keep the tank cycled.

The bacteria do not need to be fed every day so you only need to add ammonia every 2 or 3 days. If the wait time to add fish will not be more than a week, you can just add the snack dose amount 2 or 3 times in total and things will be fine. However, if the wait time to begin stocking will likely be longer than one week, do not use the 1/3 snack dose, instead, use the full ammonia dose you did when cycling. You can do this for many weeks if needed.

Keeping a tank without fish fully cycled for extended amounts of time requires doing weekly 50% water changes. Do these on the days you plan to add ammonia and do the change before adding the ammonia. If you do not do weekly water changes, nitrate will build up and KH levels will drop which can cause unwanted issues with the cycle. Further, other things the bacteria need may get used up and normally these get replaced by the new water going in. Of course, remember to do as big a water change as possible before adding fish, even 80%+ is fine.

Below is a chart illustrating what to expect in terms of ammonia, nitrite and nitrate during using this fishless cycling method. It will give you an idea of what test readings and time frames to expect.

Average Cycling Numbers

Ammonia will spike with each addition and then drop. It will drop faster with each addition. It should be under 1 ppm between days 9 and 12 of the cycle. It should be close to 0 ppm within 4 to 6 days after the second addition, and 0 ppm in 1 to 3 days after subsequent additions.

Nitrite will appear in about a week and rise to its peak somewhere between days 15-21 of the cycle. Most hobby test kits will not test this high. It will decline more rapidly than it built up and can be well under 1 ppm by days 25 to 31. It should reach 0 ppm between days 35 to 40 of the cycle.

should appear soon after nitrite does and will rise steadily until the end of the cycle. It should peak at between 35 and 45 ppm (plus any ppm of nitrate in your tap).

Some cycles can happen in a time frame shorter or longer than above, but these are exceptions not the average. While one's actual days may vary from the chart above, the pattern of one's results should still be similar.

Suggestions and Trouble Shooting

To calculate the volume of water in your tank, the most accurate way is to measure when you first fill it. The advertised volume of a tank includes the glass and assumes the tank is filled to the brim and that the tank contains no substrate, décor etc. You can measure the inside dimensions of your tank and use the Volume Calculator to determine your basic volume. Then allow between 10% and 15% for décor and multiply the basic volume by .90 - .85 to get the volume number to plug into the ammonia calculator. The smaller the tank size, the more important it is to get an accurate measurement of the water volume. Save this number as it will be needed if you have to medicate the tank in the future.

If you can add gravel or filter media from a healthy cycled tank, it will accelerate the process. The more you can add, the more it will help. If you can do this, reduce the time between testing from every 3 days to every 2 days. Test on days 3, 5, 7, 9 etc. Then reduce the every 2 day testing to every day.

Do not over dose dechlorinators especially those that also neutralize ammonia and nitrite. Over dosing will only slow the cycling process.

You added to much ammonia: Do a water change to bring your ammonia reading back to where it should be. Use the water change calculator below to know how much to change.


Your ammonia levels have not dropped by your 3rd test: Check the expiration date on your test kit. Make sure the temperature, pH and KH of your water are all within the recommended ranges. Finally, post in the forum to get help.

Your pH has dropped below 6.5 and/or the KH has dropped below 2 dg (36 ppm): Do a water change to bring up the levels. Test the tank for ammonia first as you will need to add back enough ammonia with the new water to replace what you removed. Knowing how many gallons/liters you take out allows you use the ammonia dosing calculator to know how much ammonia to add to the refill water to produce the needed ppm level. Remember, only to add ammonia for the amount changed, not for the entire volume of the tank. If changing water does not solve the problem, there are other solutions. Post your problem in the forum to get help. Be sure to include detailed information, including recent test results and the pH and KH for your tap water.

Still have questions on the nitrogen cycle? Ask them here!
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