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Author Topic: FISH-IN CYCLING WITH FISH - HOW TO DO IT  (Read 18362 times)

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Offline Sue

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« on: September 19, 2012, 03:31:18 PM »
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Fish produce ammonia, by breathing it out, and their waste decomposes to make it. Ammonia is toxic to fish – it burns their skin and gills making it difficult for them to breathe. A type of bacteria grows in the filter which turns the ammonia into nitrite (with an ‘i’). Nitrite is also toxic – it stops the fish’s blood absorbing oxygen properly, and causes nerve damage. A second type of bacteria grows in the filter which turns nitrite into nitrate (with an ‘a’). This is only toxic at high levels, and it is removed by regular water changes.
These bacteria take several weeks to grow in enough numbers to cope with the ammonia made by the fish. The process of growing the bacteria is called cycling.


There are two ways to cycle a tank – with fish and without. Both take several weeks to complete, typically between four and six weeks. Cycles have been known to be completed in less than four weeks, but this is rare. It has also been known to take longer than six weeks. The length of time taken for a filter to cycle varies quite a bit.
Several things can affect how fast the bacteria grow – the pH of the water, the hardness of the water, the temperature of the water etc. The source of the bacteria is your water supply, but your water company adds chlorine or chloramine to kill bacteria. The amount of bacteria that manage to escape being killed will also affect the length of time it takes to complete the cycle.

If fish are put into a tank that has no bacteria in the filter, they will suffer from the effects of ammonia and nitrite poisoning. It requires a lot of hard work on the part of the fishkeeper to keep the fish alive for up to two or three months while the bacteria grow. This process is called a fish-in cycle.

Alternatively, a fishless cycle can be carried out before fish are put in the tank. This involves adding a source of ammonia to the tank to simulate the fish’s waste, which encourages the bacteria to grow, so the colonies are already present when fish are added.

Seeding a filter
With both methods of cycling, a portion of mature media will help to speed the process. A piece of media from an established tank is placed inside the new filter at the beginning of the direction of water flow. If the old media is sponge, it can be cut up to make it fit.
The muck squeezed from a mature filter during cleaning will also help to seed a new filter, but this is less effective than using donated media.

Cycling with fish

A fish-in cycle can be successful if done properly. The method outlined here may differ from the way a shop tells you. This method may take a bit longer than the shop’s method, but the fish should remain healthy.

There are a few details.
•   Don’t get too many fish. The more there are, the faster ammonia will build up. The size of the tank must be taken into account. A 100 gallon tank can obviously be cycled with more fish than a 10 gallon tank. It is generally recommended to start a fish-in cycle with no more than “1 inch of fish for every 5 American gallons (19 litres) of tank water”. This means the total adult size of the fish, not what they are now – most of the fish in shops are babies.
•   Do not overfeed the fish. This is very common with new fishkeepers. Uneaten fish food will decay to make even more ammonia. Fish need a lot less food than you think. As you will be starting with only a few fish, it will not be too difficult to estimate the amount of food equal to one eye per fish. This is all you need as a fish’s stomach is about the same size as its eye. Feed this once per day. If you find your ammonia and/or nitrite going up very quickly, cut down feeding to once every two or even three days.
•   You must carry out lots of water changes. Some shops will tell you not to do any for several weeks – this is because they believe that high levels are necessary for the bacteria to grow. But they will grow at low levels; if there is even a trace more ammonia or nitrite than the current number of bacteria can deal with, they will multiply.
•   Choose the fish with care. Some fish will tolerate the cycling process better than others. Fish to avoid include neon tetras, rummy nose tetras, guppies and some corydoras. Neon tetras and guppies were once considered hardy fish, and some shops may still recommend them. However, fish farming has become big business, and the conditions that neon tetras are kept in at these farms makes them more susceptible to disease. Guppies have been very inbred, resulting in them having weakened immune systems.


The most important purchase for a fish-in cycle is a testing kit. It must include tests for ammonia, nitrite, nitrate and pH. A lot of strip testers do not include ammonia, so a separate one must be purchased. A liquid reagent test kit containing all four tests is the most cost effective. There are several makes on the market. Some testers use ppm as the measure, some use mg/l. These units are so close we can regard them as being the same thing
If you use a liquid reagent test kit it is important to wash the test tubes carefully and dry them between uses using a paper towel or tissue. Any chemical left in the tubes can give a false reading next time.

If you have bought the API liquid testers there are two quirks to be aware of.
Firstly, the ammonia test can show inaccurate results when read under fluorescent tubes and energy saving bulbs. Whenever practical, the ammonia test should be read in daylight and when this is not possible, under an incandescent light bulb or halogen light bulb (the ones that look like an incandescent bulb).
The nitrite test can behave oddly if the reading is off the top of the scale. If the liquid in the tube turns purple immediately on adding the drops, it is probably too high for the tester to cope with. It may also go an odd shade of greeny blue after mixing and standing. The way to check this is to dilute a sample of tank water with tap water, say 1 fifth tank water and 4 fifths tap water, and measure it again. If the reading is now somewhere on the scale you’ll know your tank water is off the scale.

For fish-in cycling, it is advisable to use a dechorinator that also detoxifies ammonia. These convert ammonia to the much less toxic ammonium, though it will still show up as ammonia in the test. The effect lasts about 24 hours, but as a fish-in cycle requires a lot of water changes, the ammonium will be removed before it turns back into the more toxic ammonia. The use of this type of dechlorinator will protect the fish to some extent between water changes. The ammonia eating bacteria can eat ammonium as well as ammonia so using one of these dechlorinators will not harm the cycle.
Note: ammonium is not non-toxic, just less toxic than ammonia.


When fish are put into an uncycled tank, the ammonia they excrete will build up quickly. If the fishkeeper does nothing the ammonia level  will get very high before eventually starting to fall. As the ammonia eating bacteria grow, the ammonia level will start to fall, but these bacteria turn ammonia into nitrite, so nitrite will start to rise and reach quite high levels until eventually that too begins to fall as the nitrite eating bacteria grow. After several weeks both ammonia and nitrite will be at zero.
By this time you would probably have a tank full of dead fish.

Provided you are prepared to put in some work, your fish should survive.

The key factor in this method of fish-in cycling is to keep the levels of ammonia and nitrite low. As soon as the fish are put into the tank, they will excrete ammonia. There will be no bacteria in the filter to turn it into nitrite, so the level will build up. You should measure the level of ammonia and nitrite at least once a day, preferably twice. Your aim is to stop the ammonia reading from getting higher than 0.25ppm by doing water changes to dilute it. The volume you need to change will depend on how high the ammonia is, and how fast it got that high – a larger volume changed will keep the ammonia low for longer. If necessary, a water change of 90% is fine. It is less stressful to the fish than swimming in water with a lot of ammonia and/or nitrite in it.
Don’t forget to dechlorinate the water, and get it roughly the same temperature as the tank.

The ammonia eating bacteria will slowly grow in the filter over the next couple of weeks. Once they start to eat the ammonia, you will start to see nitrite appearing when you test. Because the nitrite eating bacteria can’t start to grow until the ammonia eating ones have made some nitrite, the level of nitrite will continue to rise. You must now monitor both ammonia and nitrite, doing a water change whenever either of them reach 0.25.
For the next couple of weeks, the rise in ammonia will slow down, but nitrite will start to rise faster. Then you will find that the level of ammonia has dropped to zero, though you’ll still have nitrite – the water changes must be continued till that too remains at zero. After a few weeks you will notice it takes longer and longer for the nitrite to rise, and finally it will start to fall and you will need to do water changes less and less often. As a general rule of thumb, it takes twice as long for the nitrite-eating bacteria to grow as it did for the ammonia-eating bacteria
As the nitrite starts to be processed by the second bacteria, nitrate will start to rise. When measuring nitrate, it is important to know the amount in your tapwater (the UK allows up to 50ppm) and to take that amount into account.

When the readings for both ammonia and nitrite have been at zero for a week even though you haven’t needed to do any water changes, the filter will be cycled. This could be anything from a month to two or even three months after you got the fish.
But this does not mean you can rush out and buy a lot more fish at once. The filter has only enough bacteria to cope with the fish you already have. If you get a lot more at once, the bacteria will be overwhelmed. The fish must be added in small batches with at least a week between, and only if the ammonia and nitrite readings remain at zero between additions. It is usually safe to add the amount of fish equal to a third of the body mass of the fish you already have. Check the ammonia and nitrite levels between batches, and do water changes if they rise.

For the first six months after the filter finishes cycling, keep to a stocking limit of 1 inch of fish per American gallon (3.7 litres) – that’s the adult size of the fish, not what they are now! This will allow the tank to mature without too much bioload.

You can now start to do weekly maintenance water changes!

Ways to speed up cycling
If you can put some already mature filter media into your filter this would seed the filter, reducing the cycling time. The media could be from another tank you have, a friend’s tank, or very occasionally a shop. (Very few shops will offer this!). If you use media from someone else’s tank, make sure it is disease free.

There are products on the market that claim to instantly cycle your tank. You need to be aware that most of them don’t work, and the ones that do have an effect speed up the cycle rather that do it instantly. Some speed up the ammonia part very well, but do nothing to speed up the nitrite stage. Try one if you wish - but don’t be surprised if it has no effect.

Admin Note: A discussion of the issues associated with this thread can be read here

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