Let me toss my 2 cents in here. There needs to be a more complete understanding as to what " cycling" is and is actually doing.
I know I get pedantic about it
but I hate that we call the "cycle" a thing because the cycle is just a process and not a thing that you lose or gain. The process of " cycling" a tank is the establishing of the biological "filter" /bed that contains microbes that convert the natural toxic byproducts of living organisms to a less/non toxic state. This is the cycling process in graph form.
There is a lot of information and people now that insist that it's no longer nitrosomonas and nitrobacters that are doing the converting of ammonia and nitrites into the end nitrate product so we are probably better off just saying " Nitrifying Microbes" or " Biological filter bed" so that everybody is happy.
( No one can argue that their favorite nitrifying bacteria or currently accepted germ is not a microbe.
In the end, it doesn't really matter what is doing the process only that SOMETHING is doing the process. ) Since the biological filter bed is a living " organism" unto itself that grows and shrinks based on the amount of ammonia present in the aquarium, it is possible to lose the bed where the tank needs to go through the cycling process again. So what does a " cycled" tank mean? A cycled tank is just a tank with enough microbes in it to handle the ammonia production in the tank at that time. You can cycle a tank for 1 fish, 2 fish, 5 fish, 100 fish, etc. so that means that when the cycling process is done and if it was done with only 1 small fish in the tank, the tank is cycled only for 1 small fish. Same with 20 large fish ( or more or less fish) . At the end, there are enough microbes to handle the ammonia production of those 20 large fish or however many fish are present. Here's the catch tho, you cannot add those 20 large fish into a tank that was cycled with just 1 small fish even tho it's " cycled". There just aren't enough microbes present to handle that amount of ammonia production from the start so a new " cycling" process will occur where the existing microbes will multiply , rapidly thankfully, to catch up with ammonia and nitrite production. How fast they reproduce is dependent on the water parameters as those microbes need certain parameters to live and grow so there is no one definite answer to " how long it's going to take?" ( more on this further down the post.)
I'll argue the comment that you can't instantly cycle a tank. I did it routinely in my hatcheries. It's done by adding the nitrifying microbes from an established biological filter bed into a new tank. The key to success tho is not overloading that new tank while the bed grows more if there is now more ammonia than the bed was processing in the other tank. So for example: Take the filter, the decorations and substrate from a 10 gallon tank and place it in a 50 gallon tank and add the same fish from the 10 gallon tank into the 50 gallon tank and the tank will be cycled for that amount of fish, instantly. What these bacteria products do is basically inoculate the tank with the microbes but without a microscope and a lot of fingers and toes or a really high counting machine, you won't know how many microbes you added into the tank or how much ammonia they can handle or if they were even alive when you added them so they are a crap shoot most of the time. ( There is another aspect to using these " bacteria in a bottle" products but that's for another discussion. )
As I previously stated, the rate of time to cycle a tank is dependent on water parameters. We know that the "perfect" parameters for the microbes are: Temperature: 77-86 F (25-30 C). pH: The average pH range for optimal nitrification is 7.3-8.0. If you have temperatures reasonably below 77F, the microbes will still work but not as quickly. If you go above 86, the microbes may not die but will be highly ineffective. Above 120 F and they will die. ( This is why when you have a heater failure, you need to know how hot the water got to know if you need to re-cycle the tank.) If you have a pH below 7.3 the growth rate of nitrifying microbes slows. If you go below 6.0, nitrification is almost at a stand still. If it gets to 5.0, nitrification stops entirely. ( The good news is that at 5.0, toxic ammonia has been converted to non/less toxic ammonium so who cares if there is no nitrification because there will be no nitrites or nitrates.
) Then you need to add in that the microbes need magnesium, phosphate and other minerals as well as OXYGEN in order to function so you want a minimum KH of 80 PPM if you want to cycle your tank in a reasonable amount of time.
Back to "How long does it take?": there is no concrete number as to how long it take under "these" parameters. It takes as long as it takes. It could be days, weeks or months all depending on how you did it and what your water parameters are. Before the " fishless" cycle was in the hobby, everybody did a " fish in" cycle. If you did it right with the correct fish, they lived through the whole process. If you did it right with the wrong fish, you killed the fish.
In the end tho, you had ammonia being produced, a microbe that converted that ammonia into nitrites and another microbe that converted the nitrites into nitrates and when the ammonia and nitrites went from 0 to some higher number and then back to 0, you had a cycled tank and you had fish swimming around in the tank during the process. With the " fishless" method, you have to have an ammonia source for the cycling process to start and the amount of ammonia suggested to use is usually higher than the average fish tank full of fish produces. What will happen in the end tho is if you add more fish than the amount of microbes you created can convert quickly, they will reproduce and make more microbes. If you add less fish that need less of the microbes you created, the amount of microbes will just decrease by starvation. ( It's always better to add less fish than more at one time for this reason. You should never fully stock a tank at one time no matter which method you use.)
So that's the cycling process in a nutshell. As you can see, it's more involved than just throwing some bacteria in the water. Sadly, as wasmewasntit posted, you don't get this kind of in depth information in many stores anymore or even online from most sites but it
IS important to know so that you don't waste your time or money and don't kill a number of fish along the way. Once you know and understand this, you increase your chances of really enjoying the hobby of fish keeping because you will understand what's happening in the tank. You'll know that you can add too many fish at one time to a tank. You'll know that testing your water is a part of the hobby that you just can't avoid in the beginning. You'll understand that just because you had 10 fish in a tank a year ago but only had 1 fish in the tank for the past 3 months that you can't add back the 9 fish without a major amount of ammonia poisoning the tank in water above a pH of 6.8. In the end you will become a scientist with a really healthy bunch of tanks or as my Mentor, a certified Ichthyologist, said to me as he was teaching me all this stuff, " This is like going to school but you don't get a diploma at the end. Your diploma is a tank full of healthy fish, swimming around and you enjoying watching them."
Hope this helps.
