'Cycling', a term commonly used in the aquarium-keeping hobby.
But what does it truly mean?
Well, let's start with a simple question. What would you answer, if I was to ask you 'what does it mean for a tank to be cycled?'
If your answer is 'ammonia, nitrite and nitrate are at zero' or something similar, then well... that's not quite right. The answer will be clear as you read on.
Even if your answer is something quite different, it may be good to read on anyways.
Etymology
The word 'cycling' arose as a term quite a while back, stemming from the 'nitrogen cycle'. Because of this, the term itself has led to some confusion of what 'cycling' truly means.
Background
'The nitrogen cycle' is one of the many chemical cycles on Earth, and also one of those that involves organisms during the cycle.
Do define simply, the 'nitrogen cycle' details the transition of nitrogen from one form to another, including it's common gaseous form (N2) to other forms, such as NH3, NO2, NO3 and so on.
A simplified diagram of the nitrogen cycle, depicting the common 'waste' products we as aquarists are worried about. Image from Wikipedia.
By now, you may have noticed that some of the forms listed is what tends to be deemed as waste products in our aquarium (although many exist in ionic form instead).
Nitrogen, however, does not simply exist in its common gaseous form or as waste products. Nitrogen is incorporated into organisms as part of DNA, amino acids, proteins and many other components of the organism.
Therefore when you feed your live stock, you will be introducing nitrogen and many other elements into the tank as various types of molecules. Some of this will be incorporated into the body of your life stock, the rest will be excreted as waste products - some are what we call 'ammonia', 'nitrite' and 'nitrate'.
Now why the mention of ammonia, nitrite and nitrate specifically?
They are the part of the nitrogen cycle that we have the most interest in, when talking about 'cycling'. Now, like I said, the goal of 'cycling' is not to deplete them, though their depletion is one indication that our goal is met (confused yet?).
Why Cycle?
This is where all your questions will be answered.
Despite what the term itself would suggest, 'cycling' is not to reduce your ammonia, nitrite and nitrate (or at least your ammonia and nitrite) to zero.
The purpose of cycling, is to actually end up with enough microbes living in your aquarium, to break down harmful waste products into non-toxic molecules.
In our aquariums, as in nature, microbes is the most effective form of filtration possible. They don't just 'filter' out unwanted chemicals. They also break down all sorts of organics into much simpler molecules that doesn't foul up the water.
Microbes can reproduce very quickly (it is said that bacteria can double in population every 20 minutes, provided the conditions are right). In a quarter of a day, one bacteria can become 260 thousand. In half a day, over 68 billion.
Binary fission, the method of reproduction of (most) prokaryotes. Such as bacteria. Image from Wikipedia.
Of course, that doesn't happen, as they tend to be nutrient-limited and competition kills plenty. Oh and of course, they also end up as food for other organisms.
But what this means, is that in theory there can be plenty of bacteria in our aquariums to break down waste, and very quickly!
Cycling establishes this population of bacteria, so that when we do add live stock, our aquariums can deal with the bioload. As opposed to having ammonia, nitrite and nitrate skyrocket and kill everything.
Cycling Properly
Despite all that, cycling properly is simple.
Step 1: You need to start with two things:
1.) Bacteria
2.) Biological filter media (to house the bacteria)
Without any bacteria to start the cycle, you'll get nowhere. If you happen to have a completely sterile tank and have no bacteria introduced, then you're not going to get any bacteria to break down any waste. Once you add in your live stock, they'll overload your tank. They will introduce their own bacteria, but not enough to break down the waste.
Live rock, the most common form of biological filter media (as it can also be used in the display). Photograph taken at Majestic Aquariums.
This is also why the 'cleaner' your start is, the longer it'll take for your tank to cycle. That or you'll have to manually add bacteria, which is really not a problem.
You also need some sort of biological filter media, or essentially 'bacteria houses'. Otherwise, they have nowhere to grow. Bacteria will live and reproduce suspended in solution, but they do much better when attached to a surface (and forming a biofilm, in many instances).
Biological filter media can be anything really, from live rock to Matrix, from Marine Pure to coral skeletons, from sand to clam shells. They can live on ornaments too, by the way.
So what's next?
Step 2: Propagate your bacteria!
With bacteria existing in your system, you need to now grow them, of course. How is it done?
Simple.
What do humans need to live and thrive and grow and reproduce?
The human 'nutritional pyramid'. Obviously all this must come from different sources. The nutritional pyramid for bacteria will be different, but it's the same idea.
Well firstly, we need energy. Energy for us to do things. For bacteria, it's the same. Lucky for us, many bacteria utilizes what we regard as waste as sources of energy. And that's where we see ammonia being broken down to nitrite, then nitrate, then nitrogen gas.
Of course, just like us, energy is not enough. We can sustain ourselves, but what about reproduction? Where does all that material come from? How do we grow if we don't take in building blocks? It won't magically appear. It must come from somewhere.
That's why you also need to feed the bacteria, to let it bloom.
Think of it in terms of ourselves. We need carbohydrates for energy production, but we also need vitamins, minerals, protein, fibre, and more to actually build up our bodies and reproduce.
Step 3: Confirmation of cycle. :)
So you've let your bacteria reproduce and propagate. Great! Or, have you?
This is where the water parameter measures come in. A rise in ammonia, then nitrite, then nitrate (and the fall of the prior) would indicate that it is 'cycling', as ammonia-reducing bacteria is growing, then nitrite-reducing bacteria, then nitrate-reducing bacteria. When you first add some sort of food to your aquarium, that will produce plenty of ammonia. As ammonia is consumed and turned into nitrite, the level of ammonia decreases, whilst nitrite increases. And so on.
Now, having an efficient biological filtration system means that the bacteria should not just reduce ammonia to nitrogen gas, but reduce enough, quickly. I.e. effectively, and efficiently. There is no point if you have waste in there, and it remains there for too long. Likewise, there's no point in your biological filtration system not being able to handle the feeding regime. Either way, you'll just see a big build up of ammonia, nitrite and/or nitrate that will kill well, everything.
To test that your aquarium has 'cycled', try 'ghost feeding' your tank. If levels increase quickly and doesn't fall quickly enough, then you know you don't have enough bacteria to handle it. Think about this way - all the food you add into the aquarium will go somewhere. It will either be incorporated into your fish/corals/etc, or consumed by microbes/CUCs, or remains in the water/tank, which may then be removed through some other method. But most will be consumed. So if your aquarium can't handle a bit of ghost feeding, then you're still a ways away.
Doing It Wrong
So now that we have learnt what 'cycling' truly means, we will learn about how people do it wrong.
Let's go through each point of the cycle.
1.) No/not enough bacteria. As we have learnt, bacteria is absolutely necessary. If our tank is too clean, and we don't add any bacteria in any form, the cycle will never run.
2.) No biological filter media. Without any living quarters, there won't be enough bacteria living within the aquarium. Additionally, you run a risk of removing heaps of bacteria when doing water changes and the likes.
3.) No sustenance. If bacteria doesn't have the nutrients they need, then they will never reproduce to numbers that we desire. Many people will only add ammonia, and wonder why they never get enough bacteria. That's because well, there's nothing the bacteria can use to make more of themselves...
4.) Misunderstanding the parameters. It's not good enough just to see an increase (then decrease) of ammonia, nitrite and nitrate. Just because it has went through the cycle doesn't mean there's enough bacteria in there. You should be able to add a lot of food, and see your biological filtration system be able to take care of it. If you see ammonia increase to 0.25ppm, then drop and nitrite increase for a bit then drop, then nitrate for example, then your tank is definitely not cycled. Well yes, actually, technically it is, but remember, we do want to keep more than one fish in the aquarium. We want a large, healthy population of bacteria working their magic.
5.) Timing. If you've been given a specific amount of time to wait it out, you've been given wrong information. Cycling is not an exact procedure. You need to be flexible. If you just wait say, six months and do nothing, then by that time whatever amount of bacteria you've cultivated would have mostly died off. Conversely, if you don't wait until the end of the cycle, there won't be enough bacteria to handle your live stock. Yet, some aquariums cycle in weeks, or even days. We are dealing with living beings after all, and they will take however long they need, not how long we dictate, regardless of what we want to believe.
6.) Water Changes. Yes, that's right. Let's have a long and hard think about what happens during a water change. So firstly, water is being removed. And what is in that water? Waste right? And organics and all that. So wouldn't that be good? No. Let's remember what we are doing. We are growing bacteria. In this case, all that ammonia, nitrite, nitrate and whatever else we have in the aquarium is to feed the bacteria, and so removing it will just reduce the amount of food available for the bacteria to feast upon. That will just slow the cycle down. Everything has to be in context, and in this case what we would normally regard as unwanted, is actually something we do want.
Doing It Right
So how exactly do we do it right? Well, here are some personal opinions of mine, based on the above:
1.) Add plenty of bacteria, and to a multitude of places. Bacteria is (literally) the life source of your biological filtration system, and having them well established all over is great! Plus, obviously when crowded, bacteria, much like any organism, will lose out due to competition. In a specific area with 1000 bacteria, there's a high chance that a number won't get access to sustenance. The same 1000 bacteria, spread all over, will all (most likely) have a chance at accessing sustenance. Eventually as they all grow, it will get crowded anyways. But the question is how quickly can you make it crowded. The answer, if you have 1000 populations of bacteria growing in different places at once.
2.) Have lot's of surface area. Bacteria likes to colonize surfaces, and so they benefit from area, not volume. Having biological filter media with a large surface area (such as live rock or Marine Pure) will be highly beneficial, as that's just more space for colonization, and hence more bacteria growable.
3.) Sustain your bacteria! Like I said above, just adding an energy source is not enough. You need to actually provide sustenance for your bacteria. I like to use seafood mix. Puree it and spread all over the aquarium. Actually that's how I like to introduce the bacteria as well. :D Also, you should ghost feed your tank as it is cycling as well, just to test the waters (literally).
4.) Check your parameters regularly! It is easy to miss a huge spike that would indicate a massive amount of waste, before being consumed by bacteria. Especially make sure to double-check a short while after feeding the tank, to see how much ammonia is produced, and then how much is consumed. One way to do it is to 'feed' your tank even before you add anything to it. That way, you know how much waste is produced when there's no filtration of any sorts, from a specific amount of food. Oh and of course, after the cycle, double-check by feeding some more.
5.) Be flexible. Don't rely on a fixed schedule. Adapt to the changes of your aquarium as it cycles. If it cycles fast - great! Be ready to add your live stock soon (double-checking that there's enough bacteria to handle the bioload, of course). Otherwise, take your time, but always be ready.
Why the Focus on Bacteria?
Why do we focus so much on bacteria? Or in microbes in general?
The fact is this, we have to establish our filtration system properly before adding the first live stock (unless we don't them potentially dying). Whilst most filtration media needs no setup time, others do, and biological filtration media is the one that may take the most time (see 'Setting Up Your Equipments' in this guide). Therefore it is always best to set up the biological aspect of one's filtration system first, before setting up the others. This is especially true if you use dead/dry rock, as that may require a lot of time for microbes to establish.
Which also translates to - just because you have done establishing your microbes, may not mean that your tank is 'cycled'.
As above, the final step is to test your aquarium's filtration capacity. If it cannot handle your 'ghostfeeding', it will not be able to handle the eventual bioload. In which case, the aquarium may be missing some sort of equipment, or that you haven't gone through the microbe propagation phase correctly.
Conclusion
'Cycling' is something that is known to all of us, but is often not fully understood. Whilst eventually most tanks cycle anyways, it is always better to do it right from the getgo. Hopefully, this would have answered some potential questions that people (especially beginners) may have. :)
Remember, most importantly, cycling is not a straightforward, rigid process. You need to know how to adapt and change as necessary, to best cycle your aquarium and get the most out of it.
Once you have added your first live stock, and have no means to remove it, it is hard to improve your biological filtration system.
