What is Fishless Cycling?

What exactly is “fishless cycling” – well it has plenty to do with fish and nothing whatsoever to do with bicycles. For fish to survive in an aquarium,  there needs to be a way of dealing with their waste products and the toxins from them.  The way this is achieved is by a two-pronged approach: the use of biological filtration and regular partial water changes.

What is biological filtration?

Biological filtration in aquariums is carried out by certain types of naturally occurring bacteria which feed on the toxins generated by fish respiration, faeces, left over food, decaying plant material and so on. Fish excrete ammonia from their gills, and their solid waste decays, along with dead leaves from plants, uneaten food adding more ammonia to the water. These bacteria need somewhere to live with flowing, well oxygenated water at the right temperature and a source of ammonia. The sponges or other media inside aquarium filters provide an ideal home for nitrifying bacteria and once a filter is running in an aquarium and ammonia is present, they will colonise the filter sponges and start work.


Illustration of the Nitrogen Cycle

Beneficial bacteria feed on the ammonia, using it to grow and reproduce, and releasing another toxin, nitrite, in its place. Other bacteria feed on nitrite and convert it to nitrate, which whilst still toxic in large volumes, is considerably less harmful to aquatic life. This process whereby ammonia is converted to nitrite and nitrite is converted to nitrate is called the nitrogen cycle which is explained in more depth on this page.  To prevent too much nitrate accumulating, we must regularly change some of the water in our aquariums as well as establishing a biological nitrogen cycle. Without this, our fish would soon die from the effects of swimming in their own waste.

In the past, the nitrogen cycle was established using so-called “hardy” species’ of fish; one or two were added to a newly set up tank and the ammonia they produced caused the cycle to gradually become established. Further fish would be added every two weeks or so until the tank was fully stocked and cycled. At that time it was widely believed that fish were unable to experience pain so the only consideration was to choose fish able to survive the process whilst being exposed to ammonia and nitrite over several weeks. In more recent and more enlightened times we have come to understand that fish can indeed feel pain and that although “hardy” fish are able to survive the cycling period, they do suffer pain and discomfort, damage to their immune systems, long term increased susceptibility  to infection and a shorter lifespan. Once this was understood, fishless cycling became the preferred method.

How can I cycle a filter without any fish?

The answer to that question is quite simple: household ammonia is an ideal substitute for the ammonia produced by fish. By dosing the tank with ammonia in small amounts we can simulate the presence of fish and water containing ammonia will be readily populated by the beneficial bacteria needed to turn that ammonia into nitrite and then turn the nitrite into nitrate. Fishless cycling has numerous advantages over cycling with fish, all for just a couple of pounds to buy a bottle  of ammonia. The most obvious, and for me, most important benefit is that no fish are harmed using this method. Other benefits include:

  • The ability to add a full complement of fish immediately after the cycle is complete
  • Fewer water changes needed during cycling – these can be needed daily to protect fish if cycling fish in
  • Some time to learn the basics of tank maintenance and water testing before fish arrive
  • Time to add plants and allow them to establish
  • Time to carefully research which fish you want to stock at the end of the cycle

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