A 'cycled' tank/pond refers to an ecosystem in which things are somewhat in balance. In it's simplest form you have fish and colonies of various bacteria. The fish produce waste in the form of ammonia. The bacteria feed on the waste and produce somewhat less harmful by-products. It is my understanding that there are many (thousands/millions) different types of these nitrifying bacteria. Some fulfill the role of breaking down ammonia to nitrites. Some will consume the nitrites and produce nitrates. Within these categories there are also many types. But it is my understanding that a healthy, successful bacterial colony is a very complex community with many dependencies and symbiotic relationships. For this reason I have always been skeptical of performing a fishless cycle. It seems like the ideal scenario would be to develop the 'right' bacterial community for your fish. The problem, of course, is what to do until that colony is developed. When I've been in the position of establishing a new aquatic ecosystem my method has been:

1) introduce the lowest fish-load possible
2) monitor the ammonia, nitrite and nitrate rigorously
3) when ammonia is present I bind the ammonia with Amquel
4) when nitrite is present I protect the fish with salt
5) when nitrates are present (and ammonia/nitrites are not) I can begin increasing the fish-load

Binding the ammonia protects the fish while still making the ammonia available to the beneficial bacteria for consumption and adding salt at about 0.15% to 0.3% will prevent in intake of nitrites from the fish. So, it is based on the assumption that I can protect my fish during the 'cycling' process and that the result is a more specialized bacterial colony.

For those of you who perform the fishless cycle, do your tanks seem to be at equilibrium after adding the fish or is there a further period of adjustment that occurs due the change in the source of ammonia? In other words, do you feel as though you've developed the 'right' bacterial colony for your fish?

Hi Sonny! Yes, there are many species of bacteria that are able to perform nitrification. However, the dominant populations are frequently nitrobacter and nitrosomonas. The source of ammonia won't matter because at the end this bacteria use ammonia/nitrite and CO2 to produce nitrite/nitrate. With the fishless cycle you are dosing ammonia levels that are higher than the ones produced by the fish, thus you build up a lot of nitrifying bacteria. When you add fish, the population will be reduced to accomodate the ammount of ammonia produced by them.

Nitrifiers are slow growers, they need much longer time than heterotrophic (organic carbon consuming) bacteria. So you will need a long time to grow them, which translates in adding more chemicals to the water to control ammonia if you have fish in there.

When you add fish after a fishless cycle, it is true that you will have other contaminants in the water. Mainly organic carbon compounds. Bacteria that consume these grow very fast. Microbial communities are very complex and very resilent. They are made of groups of specialists and the fittest for a specific scenario will dominate. If the environment changes, another group may take its place.

Adding amquel and salt will also affect the bacterial community making it different of what it would be without those chemicals. So in the end, that cycle with fish is not ideal either.


If you are concerned about not having sufficient diversity in your bacteria during a fishless cycle, you can add small amounts of acetic acid to the water to build up heterotrophic bacteria too. Another important factor for the growth of bacteria in fishless cycles is the addition of nutrients and trace elements. Living things are made up of CHONPS (carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur). Adding a fertilizer like Seachem flourish will help your tanks cycle faster!

Thanks Kingyo! I appreciate the reply. I can definitely see the benefit to 'fishless cycle' and understand it can provide a jump start on the bacterial colonization of your tank. It sounds as though it is just the beginning though. Until you get your fish in there you can't really bring the bacterial populations into equilibrium with your fish.

What about the age of the bacterial community? It seems to me that age/maturity would equate to a more biologically diverse bacterial colony.

This makes me wonder about adding new fish. How long does it take for the bacterial community to adjust to the new resident? Does it depend on the age/bio-diversity of the colony?

The bacterial community is constantly changing. There was a study that followed the different bacterial species in a bioreactor treating a synthetic wastewater throughout several years. The composition of the wastewater and the reactor operational conditions were constant all this time. However, there were marked changes in the dominant species from month to month. The scientists concluded that although everything was stable, the microbial community structure varied all the time.

For biological filters, the age of the microorganisms is estimated by calculating the amount of biomass you have in the filter and dividing it by the amount taken out (water changes and filter cleanups). I have never done the calculation for my tanks... I could since I do the measurements routinely for lab-scale bioreactors. The biomass in your filters should respond very fast to addition of new fish. Organic carbon removers would grow in two or three days, nitrifiers would take probably a week to catch up. It is extremely important to avoid washing filters or changing media during this time.

I am glad I found another Koko's World member that is interested in this topic