Once you have set up your tropical aquarium the next step is to 'cycle' it. This process is the production of beneficial bacteria that will process the waste ammonia from your fish and convert it into nitrites and then relatively harmless nitrates. After adding fish to an uncycled tank the toxic ammonia and nitrite levels will rise to a dangerous high until there are sufficient numbers of bacteria to convert them into less toxic nitrates. In order to overcome this methods have been devised to encourage the growth of the bacteria. The traditional method is to add a few 'hardy' fish that will produce ammonia to encourage bacteria growth while the tank is 'cycling'. This method is very stressful for the fish, many of which die or are damaged by the process. The toxic ammonia burns the gills and leads to permanent damage. At the end of the 'cycling' you are left with fish in poor condition which you may not want in your aquarium.
A better, more humane method to cycle the tank is to use a fishless cycling method. It uses household ammonia as a source, rather than live fish, and a bacteria culture obtained from a healthy tank or purchased from an aquarium supplier.
The Bacteria Starter Culture
You can obtain a starter bacteria culture by adding some gravel, filter medium or rock from a healthy, aged tropical fish tank. If you do not have a source there are commercial products available, for example StressZyme or Hagen's Cycle. The culture will give a quick start to producing a colony of bacteria but you can also use this method without a starter culture. It will take a bit longer then to cycle your tank.
The Ammonia
Purchase some unscented, additive free 100% ammonia. Some aquarium supply shops are now selling ammonia for this purpose. If you know a chemist or science teacher they may be able to get you some pure ammonia from a laboratory supplier..
The Tank Temperature
The bacteria (Nitrosomonas and Nitrobacter) thrive best at higher temperature than is usually used for a tropical tank so during the cycling increase the temperature to between 30 and 35 C (85-95F). When you add the fish you can turn it down again.
Testing Kit
A testing kit to check the ammonia and nitrite levels is essential to test the levels and to indicate when cycling is complete.
The Fishless Cycling Method
Extra aeration is recommended while cycling as bacteria need oxygen to grow. Increase the aeration through the tank and filter if possible. You could add an extra air stone or even use an extra pump if one is available. Add your bacteria culture, then add about a teaspoon of ammonia for a small tank. Increase this for a larger tank. Allow about an hour for the ammonia to circulate then test. The ammonia level should be between 5 mg/L (or 5ppm). Add some more ammonia if the level is too low. Leave the tank alone for a few days then test again.
At this stage the ammonia level should begin to drop and the nitrite level to rise. This means that the bacteria are doing their job of processing the ammonia. Add some more ammonia when it drops to zero to give the bacteria more to work on. The nitrite level should rise until it reaches a maximum then begin to drop as the nitrites are converted into relatively harmless nitrates. Three or four days after the maximum nitrite levels are seen the nitrite level should drop to zero. The cycling is now complete.
Change about 50-70% of the water, carefully so as not to disturb the filter or the gravel bed where the bacteria are colonised. Reset the temperature to 25°C, or the recommended temperature for your intended fish and allow the temperature to stabilise for a few hours. Retest for ammonia and nitrites and if the levels are zero you can now begin to add some fish.
It is possible after cycling the tank to add all the fish in one go but this will result in a spike in the ammonia and nitrite levels which is stressful for the fish and could cause them to become diseased. adding the fish slowly will allow the bacteria to grow to cope with the added levels of ammonia excreted by the fish. Even for a fully cycled tank it is advisable to gradually add the fish. aim to take about a month to fully stock the tank.
A fishless cycling method is more efficient and produces a fully cycled tank in a much shorter time than the traditional method of using hardy fish. It is also more humane and you will not be left with unwanted, often damaged fish, at the end.
For more information about aquariums and fish keeping check out Aquariums for Sale or Fishless Aquarium Cycling
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