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Freshwater cycling


...and over the edge.
Staff member

“Cycling A Tank”

What we mean by this term, is the process of establishing an environment that will beneficially support fish life, with minimal stress to the fish and to you. The terms used here are relevant to the “nitrogen” cycle and how it relates to the aquarium.

The first item for discussion is Ammonia (NH3). All decaying matter, like uneaten food and from fish wastes, produces this. Any detectable amount is trouble for your fish, and levels of .25 ppm and above can damage fish permanently, based on length of exposure. This damage occurs to the gills, and causes breathing problems. It will damage the gills whether the fish live or die.

Secondly, the bacteria that break down ammonia produce Nitrite (NO2). This nitrite blocks the ability of blood to absorb oxygen. Even if the fish can breathe, they are not able to utilize the oxygen they take in, as easily. They can act as if panic-stricken and/or have little control over their movements. Even with short-term exposure at high levels, stress occurs and they may not be able to ward off other immune system attacks.

And finally, second bacteria forms that consume nitrite and forms Nitrate (NO3). While short-term exposure to low levels of Nitrate are not a concern, they can be if too high over a long term. Nitrates are removed with consistent regular water changes or by aquatic plant life.

A cycle is never really complete. It is established and an ongoing evolution. Keeping this “cycle” balanced and continuous is the goal of properly maintaining a tank for a healthy environment for your fish. Strive for Zero Ammonia, Zero Nitrite, and approximately 20 ppm Nitrate or less.

Ways To Cycle Your Tank

I have broken them down into three currently widely accepted methods.

(1) Using Established Bio-Logical Filter Media

This is probably the most preferred and least stressful method that will allow you to stock your fish quickly and depending on the extreme you use, to full tank load.

If you have established tanks already, without any issues, you can put filter media in them before the new set up is ready. The biological bacteria can establish themselves on it and it can be transferred to the new tank and fish added. This can be as simple as sponges on filter intakes, filter cartridges in HOB/HOT, or media in canisters. It can include substrate or décor from other tanks, such as rocks and plants.

Please ensure the tank you are drawing from, has Zero Ammonia, Zero Nitrites and less than 20-30 ppm Nitrates. It should also be a tank free of disease. Do not take media or filter squeezing from tanks you are not sure of or from a local LFS. You could be establishing your new tank with all kinds of problems. Since most bacteria will not be free floating in the water column, taking just the water will not accomplish much.

The bacteria that maintain the cycle need to be fed in order to maintain. Waiting a few days can be disastrous. If moving a tank, keep the media wet in the old tank water, aerated and for the short term only.

This method provides minimal stress to the fish, minimal work on your part, quick stocking, at or near full capacity. But you must plan ahead. It could take several weeks to establish a bacterial colony of sufficient size, on the media you move to the new tank.

(2) Fishless Cycling

Just as it sounds, you can establish the cycling environment without any fish. This method does not pose any threat to fish, establishes a large bacteria colony allowing full stocking upon completion and gives you time to decide on what fish you want.

When establishing a tank with this method, you will use a source of ammonia to initiate the nitrogen cycle. After your tank is set up, add water and treat for chlorine/chloramines. Your filtering systems and heaters should be in place and operating to your satisfaction.

Add ammonia to bring the tank to a concentration of 5 ppm. The amount you add will vary with the size tank you have. Do not be in a rush. Add small amounts and test, repeating as necessary. If you get it too high, you can drain and refill.

Now the hurry up and wait part happens. Every two days, test your ammonia level in the tank. When the ammonia levels start dropping, add additional ammonia as required to keep the ammonia at 3-4 ppm, start daily testing and test for ammonia and nitrites. Nitrites should be developing as ammonia goes away. This first stage could take 1-3 weeks.

When you see the test results showing Nitrites, start maintaining your ammonia at the 2-3 ppm range. The nitrites increasing reflect you are in to the second stage. Continue daily testing for ammonia and maintain the tank in the 3 ppm range. You will see nitrites climb so high they will be off the scale for a reading. This will continue for one to two weeks and it will seem the nitrites are never going to go away.

There will be a day where you test and the nitrites have completely disappeared, thus, the bacteria that convert them to nitrates have established themselves. When you see this drop to zero on nitrites, dose ammonia in the tank to about 5 ppm, and wait 24 hours. If at the end of that period, ammonia and nitrites are zero, your cycle has been established. Test for nitrates, and do a 75-90% water change. Pull your water down to 20 ppm nitrates and add the fish! If you have to wait to get your fish, keep the cycle established by dosing more ammonia, but you may have to do another water change before adding your fish.

(3) Fishy Cycling

This method is probably what many fall into from all the myths that run rampant out there. It is also the method that can cause severe stress or death to the fish. There are some simple techniques that make this type of cycle go easier, but take some pre-planning on your part.

The basics of this method are to set up your tank, fill, treat with a water conditioner, add some hardy fish that can handle stressful conditions, test/monitor the ammonia and nitrite levels, and do water changes as necessary.

When your cycle is established, you have a bacteria load that can handle the fish that are in there presently. More fish can be added in small numbers over a period of time, allowing for a mini-cycle to grow the bacteria levels to the new bio-load. You need to pay constant attention to your tank, testing daily, doing water changes when required, not when you can get around to it. Do not assume that you can add more fish just because reading are undetectable or low. Give the environment time to respond.

You will be captive to testing at least twice daily, doing water changes whenever ammonia or nitrites hit .25 ppm. Do not be surprised that even with a cautious approach, you lose some fish.
Bob Bishop
West End, NC


AC Members
First, rbishop, I'm still waiting for those articles you promised that would sway my opinions on fish-in cycling :look: . Second, given that (from our friendly convo) you're very biased against fish-in cycling I'd like to point out a few things about your post. It's very good, but needs to be made a little bit more fair. You almost, sort of, kind of presented fish-in cycling as an option but still adopted your stance that most people are going to mess up and wind up killing their fish. It's not that you came out and said as much. Just more of an overall tone that was in the article. Please, try to give people more credit than that.

rbishop said:
(Fishless cycling)Now the hurry up and wait part happens. Every two days, test your ammonia level in the tank.

(Fish-in cycling)You will be captive to testing at least twice daily, doing water changes whenever ammonia or nitrites hit .25 ppm. Do not be surprised that even with a cautious approach, you lose some fish
This is the first anomally that I want to point out. With both of these methods, you are a slave to testing water parameters. However, with fish-in cycling, you are only going to be doing it for about 1 week. Then, every time you add more fish, you'll be doing regular testing for another week. That's it. For fishless cycling, you're testing regularly for many, many more weeks. I'd also like to point out that, every time you add ammonia, you have to test. You'll probably have to test SEVERAL times until you get to the correct level. Why paint the testing in fish-in cycling as a negative when you're doing even more testing for longer in a fishless cycle? Also, you don't need to test twice daily. If you're starting with small numbers of fish, you shouldn't have to test that frequently. I started off testing twice daily and found that I was getting 0 ppm ammonia for at least 2 days straight. All I was doing was stressing MYSELF out while my fish looked at me through the glass and chuckled.

I think you left out some important details on how to do a proper and safe fish-in cycle. In doing so you are swaying the reader away from this very viable option. So, in an effort to make your otherwise good post a little more balanced, I'd like to add some fish-in cycling info based on my own experience cycling my tank:

Fish-in cycling can work just as well as fishless cycling while giving you fish in your tank right from the start. You also don't have to deal with dosing ammonia or doing a huge water change at the end. If you're starting a big tank, this can be a real pain in the arse, especially if you do your "final" change out and discover that you won't be able to get fish right away for another few days. Then what? Refill the tank, dose again and then repeat when you can get the fish? What if you have a 100+ gallon tank? You're talking about a HUGE amount of work for that one.

To have a successful fish-in cycle you need to start off with a small number of fish. How small depends on the size of the tank and on whether or not you can get a hold of some filter media/gravel from an established tank. Or, another option is to use biospira. I just wouldn't trust bio-spira in a fully stocked tank. It can be a bit finicky and, if it doesn't work, you'll be stuck working with a fully stocked tank...not good.

So, you can start with a few fish, test every day or every other day and do water changes as necessary. Like I said, when I was fish-in cycling my tank, I was getting so many 0ppm readings that I gave up on testing every day and tested every other day. I also wasn't going to take any chances so, whether I needed to or not, I did a small water change of about 25% every other day or every other 2 days. After you get through your first mini-cycle (usually only takes a week) you can then add a few more fish and repeat. If you're dilligent, you can start up your tank this way quite nicely and never even come close to risking losing or stressing a fish.

You can make the process even safer by adding plants. Plants don't just consume nitrates. They can also take in ammonia. You would do well to add some easy to grow plants in a fish-in cycling tank. It can really help to eliminate the stress factor. In fact most people on this site recommend that, if you're going to start a planted tank, you should load it with plants from the get-go.

So, to sum up, you can safely execute a fish-in cycle. It is not guaranteed to stress or kill your fish. The only factor that can lead to that happening is you, the aquarium owner. You get out of fish-in cycling what you put into it. It is a lot of work. However, I feel the reward of having fish right away is worth it.

So, the pros:
No ammonia dosing and the testing that goes with it.

No hunting for pure ammonia (some here have trouble finding it).

No huge water changes that need to be carefully timed with the arrival of your fish.

Fish in the tank right away. No staring at an empty tank.

Cycles begin quicker and end quicker than in fishless cycling.

You stop testing religiously when you're done stocking, usually around 3 weeks.

With fish-in cycling, you are pretty much guaranteed to get a cycle. Some here have waited weeks or months for a fishless cycle to begin with no results and no idea why it failed.

You get used to the routine of testing and doing water changes and general tank maintenance.

The cons:
You need to do work. Testing may have to be done daily, even if it is for a shorter net time than in fishless cycling.

Water changes will need to be done frequently but in smaller volumes than fishless cycling.

While you can start with fish, you will have start with small numbers and add more, slowly, over the course of a few weeks until you're fully stocked.

You do risk stressing your fish. I put it in here as a warning to start off SLOWLY. You really do need to start off with only a few fish. I started with 6 zebra danios and six cories in a 55g. That's a small number of small, not so messy fish. The smaller the tank, the fewer fish you should start with. If you follow the guidlelines above this con will not happen. Like I said, it all depends on how much you put into it.

I would just like to reiterate, the purpose of this post is not to bash fishless cycling. It's simply to point out that there are pros and cons to both methods. The pros of fish-in cycling were not really pointed out in the original post and I just felt that, in the interest of fairness, they should be presented for all to see. It's my opinion that both methods are equally viable and neither should be villified over the other.
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...and over the edge.
Staff member
My replies and references were sent to you. Through this site.

Fishey cycling is an option and one that is probably more prevalent than it should be, IMO.

I do not seewhere I adopted the stance that people will kill their fish. This article is for the newbie who knows little about setting up a tank and preparing it for fish and most likely has ended up there, after the fact.

If you are concerned about stressing the fish during the fishey cycle, you would need to be testing twice a day, if you are an inexperienced fish keeper. In the initial phases of a fishless cycle, you will not need to.

This article was submitted by me weeks ago for review by staff here at AC. I made some minor changes. I wrote the article based on what I felt the preferences should be on setting up a tank, in my desired priorities.

I have repeatedly posted the summary several times in the past weeks. It is not meant to be all inclusive and list all options. I wanted to inform people just learning or learning the hard way about basic facts. I do not debate that there are options, more detailed than I have gone into so far; maybe you will be around for the final detailed version and I would gladly collaborate with you on the fishey version.

To set the record straight, I did not make this a sticky even though I am now a mod. It was done because others felt it was informative sound advice for the new fishkeeper.

While fishey cycling is possible, for the inexperienced fish keeper, it often leads to severe stress of the fish and for many, it results in their death. Just a casual look at the posts here will reveal that.

If planned in advance, it might relieve some of the issues, but in the majority of cases, it is an after fact of concern.
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AC Members
rbishop, I'm not arguing about the potential dangers involved. I think I acknowleged as much in my last post. However, I did notice a certain lack of attention to detail to the process of fish-in cycling in your post and I'm assuming that it's due to your personal opinon on the matter.

rbishop said:
While fishey cycling is possible, for the inexperienced fish keeper, it often leads to severe stress of the fish and for many, it results in their death. Just a casual look at the posts here will reveal that.
All the more reason to not gloss over the details and really acknowlege the fact that it is a viable method to starting a tank. Many people visit this site. Many people do so after the fact, and still others do so before they get started. It is important to expose people, in an objective fashion, to the proper methods of skinning a cat (and there are many ways to do so in this case). I feel that, if you're going to post advice, you should do so in a complete, and thorough manner.

It's very important to do so because, for those that are here after the fact, the LFS has most likely told them a bunch of nonsense and they need to know what was correct and what they need to throw out. For those that come here before the fact, they need to know the merits of all of the different options before they begin planning so they can truly decide what methodology is best for them without being "guided" into one way of doing things by people on this site. Not everyone is going to have the patience to look at an empty tank. If they decide that's not the way for them, they need to be given good, complete advice about fish-in cycling with emphasis on what happens if you do it incorrectly, not "Your fish are going to die no matter what." It's just not true. That type of thing happens a lot on this site, not just in regards to cycling.

Now, you acknowlege that you were posting to inform newbies and indicated that there is a final posting to come. That's great. I'ld be more than happy to give input on the subject. However, I would like to respectfully suggest that, given your intent in the original post, you wait until it's all complete. That way you can avoid debates like this one.

And, again, why is this in the saltwater section?

BTW, just so you know, I hope I'm not giving you the impression that I'm posting with an "attitude." I'm just trying to be frank. I don't see it like this:

:argue: I see it more like a friendly debate, a dance if you will: :dance2:

And, after reading all these long posts, I usually feel like this: :thud:

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AC Members
if you do not have access to media from another established tank and you are doing either fishless or with fish cycling, will the beneficial bacteria grow without a 'seed'?

so if I start adding ammonia to the tank, or throw a raw shrimp in there will the bacteria just show up?


...and over the edge.
Staff member
Yes, since they are naturally there.


Registered Member
?? Cycle Complete

Is my cycle complete? I seeded my 38G with a filter from my 7G & some gravel. Then added my black tetras. I have tested the water everyday for the last week. The currect results are listed below. How long do I need to wait before I can move my tiger barbs out of my 7G? Tank to small. New to this forum & new to a larger aquarium.

pH 7.8
Ammonia 0
Nitrite .25
Nitrate 5.0


AC Members
The cycle isn't complete until your nitrites are gone and your nitrates are a bit higher. Then you can do a water change and add one fish at a time.
Nitrites and ammonia test 0 on established aquariums.
Basically the cycle works like this:
The first type of bacteria turns ammonia into nitrite. When this colony is established your ammonia will rapidly drop and your nitrites will rise.
The second type of bacteria turns those nitrites into less toxic nitrates. Nitrites will drop and nitrates will rise. Once your nitrites are no longer present but nitrates are you do a water change. Now your tank is cycled.

Nitrates are always present in most freshwater aquariums. They are dangerous is large quantities but usually harmless. Plants can do a good job of lowering nitrates, and water changes are also necessary so they don't reach above about .40 ppm.

Over course this is overly simplified, but it should be fairly easy to understand.

You only want to add one fish at a time because if you add several at once ammonia will rise faster than the bacterial colony can grow, which can cause a mini-cycle.
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One bulwet left!
Hi hankn. I'm sorta a newbie myself. Do you have any fish or plants yet? If so I would read up on the original post of Ghostshrimp 55 on this thread to help you with water changes and "Fishy Cycling." The fishstore should have siphons made especially for changing water and cleaning gravel. And you should add something to neutralize the chlorine and any impurities in your tap water. There are several brands. I use Aquasafe and add it right into the bucket of fresh tap water that's the same temp as my tank before adding it to the tank.


AC Members
Quick question, can I start to cycle a tank without plants and ornaments? And then add them sometime before getting the fish?