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Fishless Cycling
by Tim Craig (prov356)

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"It takes X hours for my ammonia to go down and then another X hours for nitrite to drop. Is this good? Am I cycled?"

--Do not test your aquarium water until 24 hours have passed since you added an ammonia dose. You are over-analyzing things. How long it takes for ammonia and nitrite to be converted is not important. The only thing that is important is that both are fully converted within 24 hours. This is just a way of measuring whether a sufficient colony of bacteria has been established. Hobbyist testing has shown that if the ammonia is fully converted within 24 hours, then you are ready for fish. Whether the ammonia is converted in 8 hours or 12 hours, etc. is not meaningful when fishless cycling.

Do not over think it.

"My ammonia is zero, nitrite is 2ppm, and nitrate is about 10ppm. Since I have a nitrate reading, am I cycled?"

--The short answer, no. There is often a temptation to test for nitrate to see if cycling is almost complete because the nitrite tests are not showing any change. Understandable, but know that many of the nitrate kits work by first converting any nitrate to nitrite, then determining total nitrite. Therefore, if nitrite is in the water, results will be skewed. Testing for nitrate before nitrite has dropped to zero can only complicate and confuse. The determinants for a completed cycle are ammonia and nitrite tests that read zero.

-Yes, you may have nitrate because there is bacteria present converting nitrite to nitrate, but until they build enough to convert all of the nitrite, your aquarium is not considered cycled. Be patient and test for nitrate only after ammonia and nitrite read zero. The nitrate test is done only to determine the level of water changes needed before adding fish, not to determine if the aquarium is cycled. So, do not try to use it that way.

"What does 'ppm' mean?"

--It means parts per million. Most ammonia, nitrite, and nitrate test kits give results in parts per million. Think of making iced tea. If you add more tea, it gets stronger tasting because there is a higher concentration of tea. If it is too strong, adding water will dilute it and make it weaker. The same thing applies to working with 'ppm'. If ammonia is too high, adding water will dilute it and make it weaker. So, if you take one cup of water and add ammonia until the level reaches 1ppm, then dilute it by adding another cup of water, the ammonia level now has to be 0 .5 ppm. You have cut the concentration in half.

-Alternatively, if you start with a cup of water with a level of ammonia that is 1ppm, and then add the same amount of ammonia again, the ammonialevel or concentration now has to be 2ppm. Keep in mind that hobbyist test kits may or may not reflect this, and this sometimes can confuse. Hobbyist kits will give you a ballpark reading only.

-The fact remains, if you take an aquarium with a level of ammonia that is 6ppm, and then do a 50% water change, it must reduce the level of ammonia to 3ppm. That is the important takeaway. If you do not get a 3ppm reading, then either it was not 6ppm to start with or it's just difficult to accurately discern the level of ammonia from the color cards. That is why I suggest cycling to a range of 1-2ppm. Do not frustrate yourself trying to get precise
readings from a test kit that was not designed to be precise.

"What if I'm going to initially stock my aquarium with a full load of adult fish?"

--Then simply use an ammonia level in the range of 2-3ppm to cycle, instead of 1-2ppm. Try to use an ammonia level close to 3ppm, but do not go beyond 5ppm. In addition, be aware that you will need to do more frequent partial water changes to keep the byproducts of cycling down during the process.

"Do I really need to do water changes while cycling?"

--There is some debate about this. I cycled many aquariums successfully before I had ever heard of this practice. So, need to do? This has not been my experience, but there can be many variables. Some of these variables may force the necessity of the water changes.
-I do know that cutting the nitrite level towards the end of cycling via water changes can help in some emergency "I need the cycle to be done now" situations. (See below.) I'd also suggest that the water changes can help to keep the cycling time as short as possible and, more importantly, help to ensure that the byproducts of cycling don't reach levels that can inhibit cycling.(5)

-Just be sure to keep the water changes down to no more than 1/3 of the aquarium volume as noted in the steps to fishless cycling listed previously.

"How long can my aquarium run without fish before losing my nitrifying bacteria?"

--Some variables make it impossible to give an exact length of time. I have left aquariums with well seeded filters running for a few weeks empty and then added fish and saw no ammonia or nitrite spikes. In my experience, you do not lose the bacteria as quickly as you would think or what is often portrayed in the forums. Moreover, even if the numbers of beneficial bacteria have declined, they bounce back quickly. Remember, there are organic solids in the
filter and you have heterotrophic bacteria breaking down those organic solids into ammonia, which is a food source for your nitrifying bacteria. This can keep a biofiltration system going for a long time.

-As a simple test, just add 1-2ppm of ammonia to the aquarium. Test for ammonia and nitrite after 24 hours. If ammonia and nitrite are zero, then the aquarium is ready for fish. If not, keep testing daily and follow the fishless cycling steps above. It should only take a short time to ensure that the system is fully cycled and ready for fish.

"What's a biofilm?"

--A biofilm is formed when bacteria adhere to surfaces in layers. They adhere to surfaces because that is where nutrients congregate.
A mature biofilm can be hundreds of layers thick. Within this biofilm are different types of bacteria, not just the nitrifiers.

You can also find heterotrophic bacteria that break organics down into ammonia, and denitrifying bacteria that convert nitrate to nitrogen gas. The layers that make up a biofilm have organization and structure. Water channels flow through these layers. The different types of bacteria are each found in all layers. A biofilm protects bacteria from protozoa, various predatory algae, and predatory bacteria.(7)

Common problems

"My fish are on the way and my aquarium isn't cycled yet! What do I do?"

--I have seen this question posed a few times and have been in this situation myself. I am assuming the finish line is within reach, meaning ammonia has been testing zero, but you are waiting for nitrite to drop. If your ammonia level hasn't dropped, then you need to either postpone the shipment or make other arrangements to house the fish temporarily. If you are close to the completion of cycling and you need to accelerate the process, then here is
what may help.

-Stop the ammonia additions for now. Do a series of partial water changes to try to get your nitrite level down well below 1ppm.
Test the next day to see if it has finally reached zero. If not, wait another day and test again. You can also perform another water change to get nitrite levels down to barely perceptible, meaning .25 or so. Test again the next day.

-Ok, so let's hope that you have now gotten a zero nitrite reading. What you do next once levels are zero depends on how many days until the fish arrive. If you still have a few days before the fish arrive, then add a small amount of ammonia, say 0.5ppm or so. If both ammonia and nitrite read zero the next day, then you know you have at least cycled to 0.5ppm. Use your judgment at that point as to whether to continue pushing some ammonia to build the bacteria colony or leave well enough alone. It is better to leave well enough alone rather than risk introducing new fish with perceptible levels of ammonia or nitrite present.

-I have found the bacteria colony will adjust rather rapidly, if need be, to the ammonia load of the fish. In addition, you really should not be feeding those new arrivals on day one anyway. Moreover, if you have gotten them from a professional breeder, they have probably been fasted for a couple of days before shipment. Therefore, the initial ammonia load from the fish should be low to moderate.

--In summary, to handle this situation you want to cut your nitrite level way down with water changes and hold off on adding ammonia to see if you can get nitrite levels down to zero. Then make some judgments from there based on the amount of time left before the fish arrive. Increment the ammonia additions in tiny steps to bump up the bacteria colony while allowing a couple days buffer between the last ammonia dose and the arrival of fish to ensure
ammonia and nitrite zero out before they arrive.

-Do not worry so much about nitrate at this point, as you can deal with it after the fish arrive. Elevated nitrate levels are not a short-term threat to fish health. You can always do the water changes to deal with that after the new fish arrive.

-If you are never successful at getting nitrite to zero out, do not panic. Keep nitrite low with water changes, go very easy on feeding, and the nitrite will zero out very soon. I would recommend not feeding at all until nitrite zeros out because I believe that you will see that happen in a very short period. The fish will be fine without being fed during that time. Elevated toxins are a much greater risk to them. Also, consider using one of the products that claim to detoxify ammonia and nitrite.

-Now someone may ask, "can't I do this even if fish aren't on the way just to accelerate the process and be done?" Yes, you can, but what this shortcut method is intended to do is help to ensure that the aquarium is fully cycled, even minimally. If you are intending to cycle to 2ppm and adopt this method to cycle to 0.5ppm, then you have finished sooner, but you have not built up the same amount of bacteria that you would have if you had just gone ahead and cycled to 2ppm. You would then need to bump up the ammonia additions until you reached 2ppm and compare the process times of both methods. I am not certain that you will see a huge difference.

"My ammonia level hasn't dropped and it's been over 10 days now. Is my cycle stuck?"

--It is probably not 'stuck', but there are some things that can inhibit the cycling process. Check the pH of the aquarium water. Nitrifying bacteria are inhibited at a low pH.5 If pH is low, say 6 or below, then raise it up to 7 or higher using sodium bicarbonate (baking soda). Get the KH up to at least a level of 4 - 5 or so and pH will rise and stabilize. The cycling processes should now continue.

-Another thing that can inhibit the establishment of a beneficial bacteria colony is changing or making adjustments to filters, filter media, décor, or just about anything else in the system. If possible, leaves things be. Accept that there will be time for changes later. I would recommend not making
any major changes until at least 45-60 days after cycling is complete.

-Low temperatures can also inhibit the cycling process.(5) Don't try to save on electricity by cycling an aquarium in a cold garage or basement without a heater. Get the temperature up to at least 70 degrees Fahrenheit, and higher is better.

-Certain heavy metals have also been known to inhibit nitrifying bacteria.(5) If you suspect that this may be a contributing factor, then use a product designed to detoxify heavy metals.

-Lastly, ultra-violet light can kill nitrifying bacteria. Be sure to turn off that UV sterilizer during the cycling process. If none of these is an issue, then just wait it out a bit longer.


--Fishless cycling is a safe and easy method of getting your new aquarium system off to a good start by providing a healthy environment for your new fish. In the process of learning about fishless cycling, you have also learned a few things about the beneficial bacteria that are a necessary part of every successful aquarium. You can use this new arsenal of information to help make you a better fish keeper. Your fish will thank you.
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1. Walstad, Diana L.; Ecology of the Planted Aquarium, Echinodorus Publishing, Second Edition, 2003, p. 20, 22.

2. Robert M. Durborow, David M. Crosby and Martin W. Brunson; Ammonia in Fish Ponds; Southern Regional Aquaculture Center, June 1997.

3. Terry D. Bartelme; Short Take: Stress In Fish, Part II: Why You Should Care about Stress in Fish;
Advanced Aquarist; Volume III; Sept 2004.

4. Report to the Director Idaho Fish & Game; Fish Health and Stress Related to Transportation; Appendix 3.3 Idaho's Anadromous Fish Stocks: Their Status and Recovery Options; 5/1/98.

5. The Water Planet Company; Nitrification and Denitrification; 2010.

6. Ammonium hydroxide; Wikipedia, The Free Encyclopedia; March 2011.

7. Walstad, Diana L.; Ecology of the Planted Aquarium, Echinodorus Publishing, Second Edition, 2003, p 69-71.
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