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Bernie Bernie is offline
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Lightbulb Info: On Cycling A New Tank


There are 2 ways to Cycle a new tank, either with fish, using their waste products to help create ammonia or without fish (which is called Fishless cycling). You must always cycle a new tank!

What is cycling ?:

It’s a little complicated, but it’s very important to know when you own an aquarium. With a new tank/filter, you do not have enough beneficial bacteria to remove ammonia from the water, so if you aren't careful your fish could die. Cycling a tank is basically breeding a bigger colony of beneficial bacteria to convert the amount of waste your fish produces and any decaying food. There are two types: one converts ammonia to nitrite, and the other converts nitrite to nitrate. Buy a test kit to monitor the levels of these three to see how far along your cycle is, and to make sure they don't reach dangerous levels for your fish along the way. While the tank cycles, you should see a spike of ammonia and it will go down, then a spike of nitrite and it will go down, then nitrate will start to increase. Do not rinse the gravel or anything in the filter in tap water or the chlorine will kill the beneficial bacteria.

The Cycling process has 3 phases:

1st Phase:

The cycle starts as soon as there are fish in the tank or you start feeding the tank (if using fishless cycling). The waste products, & any uneaten food, break down into either ionized or unionized ammonia, which usually starts to form around the 3rd day after putting fish in the tank. The ionized form, Ammonium (NH4), can be present in a pH below 7, and is not usually harmful to fish. The unionized form of ammonia (NH3), exists in a pH of 7 or above, and this ammonia is highly toxic to fish & any amount can be dangerous, but if levels reach 2 ppm, the fish start to stress & will suffer. P.W. changes are the best way to stop high levels of ammonia forming.

You usually need to test for ammonia from the 3rd dy. & everyday from then on until you see it start to drop & then you continue testing every 2nd dy. until it remains at 0.

2nd Phase:

This is where the ammonia starts being eliminated by the “Nitrosomonas” bacteria, this process is what produces the nitrite, which is also highly toxic to fish. Nitrite levels of 1 ppm can be very harmful to some fish.

Nitrite usually appears at the end of the first week after adding fish, (but can take longer to start), which is when you should start testing for nitrite, & keep testing every 2nd dy until it reaches 0.

Again p.w.changes will help keep nitrite levels down.

3rd Phase:

This final phase of cycling (which can happen very suddenly), is when bacteria called "Nitrobacter" bacteria change the nitrites into nitrates, which in moderate levels is not too harmful to fish, an acceptable level is 5 ppm, but can be as high as 30 ppm before it starts harming fish (but it isn't a good idea to leave it that high if it does get to that level). In a cycled/established tank you should normally get a nitrate reading that is between 5-10 ppm.

P.W.changes will keep levels down if they get too high, & once the tank has been established you only need to check the nitrates every 3 months or so.

Once you have a constant reading of ammonia = 0 nitrite = 0 nitrate = 5 ppm (but nitrate may be a bit less/more) for a couple of dys, you're tank has finished cycling.

The different ways to cycle a tank:

Cycling with Fish:

When setting up an aquarium, buy some hardy fish (definitely not neons) to get the cycle started. These fish are often called "Starter Fish" & the purpose of these fish is to provide ammonia through respiration, fish waste, and decaying food. The ammonia allows the first set of nitrifying bacteria to colonize and to initiate the cycling of the tank.

During this time of cycling, ammonia and later nitrites will spike up to dangerous levels for the fish, so after 3 dys of adding the fish, you need to monitor these levels daily & do partial water changes every 2 to 3 dys (it may be needed every dy if levels get too high, also cut back on feeding or stop feeding for a day of two). Some fish may not survive the cycling process, so it is best to be prepared for some losses.

The cycle is complete as soon as ammonia and nitrite levels are no longer measurable, consistantly remain at 0 and you have a nitrate reading as mentioned earlier. This form of cycling takes anywhere from 4-8 wks.

Fishless Cycling:

All you need to do is set up the tank with at least gravel, a filter and water and preferably a heater to maintain a temperature Of around 80*F as bacteria grow faster in higher temps. (but it is not absolutely necessary). Add cheap fish flakes daily and siphon every two weeks just as if you had fish in the tank & keep monitoring the levels of ammonia, nitrite and nitrate.

Once your nitrogen cycle is fully established, do a series of water changes over a few days (still feeding) and then add your fish. This way you can add more at a time and none of the fish will be harmed from ammonia or nitrite levels, and it's much less work and stress on yourself.

Seeding the tank can speed up this process. (Seeding basically means the introduction of existing bacteria colonies into a new tank). The decaying food will provide ammonia for these colonies to settle and grow in the new tank. This method also takes 4-8 weeks.

There is also a second way to do a fishless cycle which requires a bit more effort, which is:

Using Pure Ammonia to Cycle:

Instead of using fish food for ammonia production, you can also introduce pure ammonia to the tank.

After the tank has been set up, add 5 drops of ammonia per 10 Gallons into the water on a daily basis.

Ammonia will rise to 5 ppm and higher. As soon as you can measure the nitrites, reduce the ammonia to 3 drops per day. Nitrites will rise to similar levels. Keep adding 2-3 drops until the measurements of ammonia and nitrites come out with 0 ppm. The tank has then completely cycled.

With the fishless method, when the fish are introduced into the tank there is a risk of creating by-products such as phosphates, which occurs by decaying food. The ammonia produced might not be sufficient to create enough bacteria colonies to hold the fish when they are introduced, and this will trigger another growth of bacteria with the spikes in ammonia and nitrites.

These re-renewed spikes however will be much shorter and less intense compared to the initial ones experienced during the primary cycle. Consequences for the fish are minimal, making this at least fish-friendlier. This method is usually quicker taking approx. 2-3 wks, but it can vary.


A tank may become cloudy during cycling which is normal, it is usually caused by a bacterial bloom as the bacteria establish/colonise and usually will clear up on its own.

The tank needs to be well oxygenated as the bacteria require oxygen.

If using ammonia cycling:

The ammonia used should be free of any perfumes and additives, but just in case, use act.carbon to remove any traces of them after the tank has cycled.

Do not treat the water with conditioners that remove ammonia.

Water changes are only necessary if the ammonia and nitrite levels are too high, which should only occur if more than 5 drops is used per 10 Gallons of water.

After stocking your tank with fish, general maintenance of the aquarium is all that is required. The bacteria will adjust to the fish load and if you plan to add more fish the bacteria will need to adjust again.

Remember that a tank has cycled if ammonia and nitrites are back at 0 ppm. At this time you can stock the tank with fish. If no fish are introduced, the bacteria will still need to be fed.


References: Bernie, LittleHippyGirl, Algone.com, freshaquarium.about.com

Last edited by Bernie : at .

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