Biological Cycle in the Aquarium

What is the biological cycle?

Although this cycle can be found under many different names they are all referring to the same process. It is often called the nitrogen cycle, new tank syndrome, cycling or nitrification process. Basically the biological cycle is adding beneficial bacteria to your aquarium via the filters, décor, substrate and even the tank glass so that ammonia and nitrites are converted to nitrates which can then be removed by water changes. It is very important that no ammonia or nitrites are present in the water as these are harmful to the fish and will eventually kill them if the fish are faced with them over a period of time.

The cycle starts with ammonia being present in the water, this is produced by fish or rotting vegetation and the presence of ammonia in the water is the first stage of the biological cycle in the Aquarium. If the ammonia is left unchecked it will burn the fish and weaken their immune system so there are special bacteria that deal with this called the nitrifying bacteria or nitrosomas.

The nitrosomas will convert the ammonia into nitrates, these are less toxic to the fish but still harmful. Once the ammonia has been converted into nitrates you are on the second stage of the cycle. The nitrosomas do occur naturally in water, what you are trying to achieve is to build up a large enough colony inside the aquarium and especially inside your aquarium filters that can cope with the amount of ammonia that is being added to the tank by the fish producing waste. Once this colony is large enough another group of bacteria will colonise your aquarium and these are known as nitrobacter, they convert the nitrites into nitrates which can reach an acceptable level before being detrimental to the health of the fish ( normally below 40 ppm). The levels of ammonia, nitrites and nitrates can be tested and this will be covered in a later next section of this article.

Once the nitrobacter have built up they are capable of converting all of the nitrites in the water and you have reached the third stage of the biological cycle. Now you will have two sets of bacteria that are capable of dealing with ammonia and nitrites in the water but as fish are added the ammonia levels will rise as more waste is produced , this means that the bacterial colonies need more time to grow so that they can handle the extra bio-load. This is why it is always important to add new fish slowly and do not over stock your tank , this will put too much pressure on the bacteria to perform efficiently.

How do I start the cycle?

All in all cycling the aquarium can take up to 4-6 weeks from start to finish, you have to be patient during this time, rushing the process will mean that the tank will not be cycled properly and it will not be ready for fish when you are. There are basically three methods for introducing ammonia into the aquarium but all three methods have the same requirements. The nitrifying bacteria and nitrosomas need a base for there colonies to build on, this is supplied with the media inside your aquarium filters. The filters must be rated for the size of the aquarium, if they are too small they will not cope when the fish are added. The water must contain oxygen, depriving the bacteria of this will slow down their colonisation and they will not function properly.

The first method used for starting the cycle is by adding a few hardy fish. I prefer not to use this method as it can put undue stress on the fish but some keepers perform this method all of the time. Heat the water to the required temperature and turn on the filtration system. Hardy fish that can be used are Platys or Mollies, only add 2-3 to kick off the cycle. In about 6 weeks their waste should have provided enough ammonia to support the nitrifying bacteria which in turn will provide enough nitrates to establish your colony of nitrosomas. Test your water on a regular basis to ensure that you have readings of zero for ammonia and nitrites, if so your aquarium is cycled and ready for use.

Method 2 is to add ammonia directly into the tank, this should be pure ammonia, never use the scented variety as this will foul your water, ammonia chloride works just as well. Using a dropper so that yo can control how much you add, keep adding drops of ammonia until you get a reading of 5 ppm. The nitrite reading will be zero initially but as you start to get a reading the ammonia added to the tank can be reduced to 3 ppm. Keep repeating this process daily until there is no longer a reading for ammonia or nitrites. The tank is now cycled.

The third method is to add fish food or frozen prawns to the aquarium, as they decay they will produce ammonia. This is a very slap hazard method as you have no control as to how much ammonia is being added to the water but the end result should be the same.

How do I test my water for ammonia etc.?

There are many testing kits available to buy that will test your aquarium water for ammonia, nitrites and nitrates. You can buy each kit separate but this will work out more expensive so stick with the kits. They are available in liquid form or test strips. The liquid tests are easy to use, you simply add the instructed number of drops to a test tube containing your aquarium water and wait for a couple of minutes to get your reading which is obtained by holding the test tube against a colour chart. The testing strips do the same thing, they are simply dipped into your aquarium water and will give results for a number of tests in one go. The only drawback with these is that they are not as accurate as the liquid tests because they cover a broader range with each strip, I much prefer to stick with the liquid variety.

Failing this many local aquatic stores will test your water for you, this is an easy option but beware, some of the staff can give you misleading information, it is much better to test the water yourself.

Hopefully you will now understand the biological cycle in the aquarium and feel confident about cycling your own aquariums!