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Cycle Your Tank! A Complete Guide For Beginners

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Nov 28, 2006
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Teesside, UK
Fishless cycling

Silent or plant cycling

Fish-in cycling

Cycling is the term we use for the process of growing beneficial bacteria in a fish tank, and a tank which contains these bacteria is called a cycled tank.
The name 'cycling' comes from the nitrogen cycle.


Fish excrete ammonia as their waste, but it is poisonous to fish. In a cycled tank there is a colony of bacteria which 'eats' this ammonia and they turn it into nitrite. This is also poisonous. In a cycled tank there is another colony of bacteria which 'eats' nitrite and they turn it into nitrate. In nature more bacteria turn nitrate into nitrogen gas, but these bacteria rarely grow in a fish tank so we do water changes to remove the nitrate.

Because these bacteria multiply slowly, it takes several weeks to grow enough of them to remove all the ammonia made by a tankful of fish. If fish are added to a tank which does not have enough bacteria they can suffer the effects of ammonia and nitrite poisoning; they will be permanently damaged and will probably not live as long as they should, and they may even die.

There are bottled bacteria products in stores but these do not instantly cycle a tank. Some have no effect while others only speed up a cycle. If you wish to use one, ask in the forum for recommended products.
Mature media from another tank can also speed up a cycle, but be careful that the mature media does not contain any diseases.

There are two methods of cycling which grow the beneficial bacteria, and a plant method which relies on plants to remove ammonia rather than bacteria.

Fishless cycling

This method of cycling involves adding ammonia to the tank to grow the bacteria before fish are put in the tank. Bottles of ammonia or ammonium chloride can be bought for this purpose. Bottles of ammonia must be pure ammonia and contain no detergent, surfactant or perfume. If pure ammonia or ammonium chloride is not available, it is possible to use fish food to make ammonia but this is not as effective as using ammonia from a bottle. Because food has to rot down first there is no way to know how many bacteria have grown so fish have to be added slowly, a few at a time, as in the other methods.
The method we use on this forum is different from fishless cycling methods found on other sites. Because too much nitrite can stall a cycle, our method was written so that ammonia is only added when certain targets have been reached, and this stops nitrite getting high enough to stall the cycle.

This describes in detail how to carry out a fishless cycle

While carrying out a cycle, it is often necessary to refer back to the method, and there is a simplified version which contains just the method itself to make checking on the cycle progress easier.

Silent or plant cycling

Live plants need nitrogen to grow and aquatic plants can get their nitrogen from ammonia excreted by fish. They take up ammonia faster than the bacteria can.
A silent cycle involves planting the tank heavily with fast growing plants. Just a few slow growing plants won't be able to use all the ammonia made by a tankful of fish; there must be a large number of fast growing plants. If only a few plants or slow growing plants are wanted, they will not be able to remove all the ammonia made by a tankful of fish; in this case it is better to carry out a fishless cycle and add the plants once the cycle has finished.
With this method, the tank is planted then left to allow the plants to establish themselves. Once signs of active growth appear, fish can be added a few a time, testing the water daily for ammonia and nitrite. If they both stay at zero for several days, the next batch of fish can be added; again the water should be tested daily to make sure the plants are removing all the ammonia made by the fish.
There are special plant substrates on the market, though these are not usually needed except for hi-tech planted tanks. Some of these plant substrates are known to leach ammonia into the water for several months. With these substrates, fish cannot be put in the tank until ammonia has dropped to zero.

This describes in detail how to do a silent, or plant, cycle

Fish-in cycling

This method involves putting fish into a brand new tank and using their waste ammonia to feed the bacteria. Decades ago, this was the only method used to cycle a tank but it has now fallen out of favour as it can damage the fish. But many new fish keepers find themselves doing a fish-in cycle, often because the fish store did not tell them about cycling. If done properly, it is possible to do a fish-in cycle with minimal harm to the fish.
Fish-in cycling means finding a balance between allowing the levels of ammonia and nitrite to get high enough to grow the bacteria quickly, and keeping ammonia and nitrite low enough so they don't harm the fish. It is preferable to keep the levels low so that fish are not harmed even though the cycle takes longer.
Fish-in cycling involves adding a few fish and testing the water for ammonia and nitrite every day. Whenever either of them gives a reading over zero, a water change is done to remove them. Once both ammonia and nitrite have remained at zero for several days, the next batch of fish can be added, again testing every day and doing water changes if the readings are above zero.

This method goes into more detail on how to do a fish-in cycle while keeping the fish safe

It is possible to minimise the effects of ammonia and nitrite, though the ways of doing this are more complicated. Those fish keepers who wish to use these methods can find them here

Acknowledgements - Chad for the use of the cycling diagram.
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