There are, as is the case with every aspect of this hobby, numerous ways in which to accomplish a given task. Starting a biological filter is no different. As everyone has their personal preferences or method to accomplish this, I will do my best to list and explain the methods known to me. Before I do so, for those unfamiliar with the term "cycle", I will explain it as I know it.
Cycle = The Bacterial Nitrogen Cycle, also known as the biological filter, since it is a specific group of bacteria (aerobic, which means oxygen is present) that colonize our aquarium's and convert the ammonia that all life in our aquariums produce into nitrites, another group of bacteria will then use the nitrites and convert them into nitrates, normally this is the end of the line for ammonia and where plant life (algae) takes over and uses the nitrates as their food source. But, there is another group of bacteria (anoxic / anaerobic, which means low or no oxygen present) which will colonize the oxygen free areas of our aquariums such as the substrate and use the nitrates as an energy source thereby converting the nitrates into nitrogen gas, which will then bubble up harmlessly out of our aquariums.
Without these bacteria, ammonia levels would remain at such levels that life would be impossible to maintain in our aquariums, as such, it is these bacterial strains that we need to allow the time and an initial source of ammonia to spark their growth. The only differences in the methods of starting this cycle is in the type of initial ammonia source and what to do with the resulting nitrates.
It is also important to note that our aquariums are always in a state of cycle, in that ammonia is always being produced and always being converted by the biological filter (the bacteria). Which is why even mature aquariums must still stagger any livestock additions to give the biological filter time to grow to the new larger bioload placed upon it.
There is really only two concerns with starting a bio-filter, what source of the initial ammonia to be used, and then what to do with the end product of nitrate. But before I get into those areas, there are methods / products available that can skip the "start from scratch" methods normally used.
LIVE ROCK & LIVE SAND - If the live rock you purchase has been properly cured and has been maintained properly as well as using live sand, either purchased or taken from an established tank, then both rock and sand will most likely have sufficient numbers of the needed bacteria present to be able to safely maintain the tank for its first inhabitant. Which by the way should have been in a quarantine tank prior to its introduction to the show tank. A quarantine tank should be the first tank you set up at least a month or more before setting up your show tank.
For more in depth information please read - RAPID CYCLE METHODS.
But now you have decided to use the old tried and true method of starting a biological filter, as I stated earlier, you must then decide on what ammonia source to use and then what to do with the resulting nitrates.
PURE AMMONIA - You can buy a small bottle of pure unscented ammonia at any supermarket and add 3 drops of this ammonia to each 50 gallons of water. Note I said DROPS.
FISH / SHRIMP MEAT - You can instead also drop into the tank a very small piece of raw shrimp or fish meat and allow it to rot into ammonia, all it takes is a very small piece no bigger than a half inch cube.
I am sure there are other sources of ammonia, such as anything that is dead and will rot, such as fish food or the neighbors cat. I only suggest either pure ammonia or seafood meat.
Note on the use of live fish to act as an ammonia source, Dont do it! Not because I am a tree hugging save the whale type, but because any and all fish need to have gone through a quarantine period outside of the show tank. To just add any fish no matter what its purpose is, right into the show tank is only asking to introduce the Ich parasite to your show tank. Its not worth the risk.
Now that the initial ammonia source is in the tank, now its time to wait and let the bacteria multiply and do their thing. This process can take anywhere from two to six weeks, during that time the ammonia levels will rise sharply, peak out and then start to fall towards zero. At the same time, nitrate levels will begin to rise. I recommend that during this time, you leave all tank lights off so as to avoid a massive algae bloom. Since nitrates and light are what will fuel what is normally a massive algae outbreak, what are we to do with all those nitrates?
So now all the ammonia has been converted and you can no longer detect it on your test kit, more than likely you now have dangerous levels of nitrates as well as the start of a huge algae problem, the most popular method is to perform massive water changes to bring the nitrates down as close to zero as possible, but what normally happens is that while waiting for all the ammonia to disappear, algae has had time to take over the tank, if you think about how a tank is normally allowed to cycle, I get the impression that it is a process that we have no control over and can only stand there and watch our tanks become a mess, then and only then are we allowed to do something about it and spend the next few months trying to fix it and usually end up shocking the tank and upsetting things even more. Well, this is where I part company with the rest of the hobbyist and feel that a new tank's cycle can be kept under control.
This procedure goes under the assumption that you have and use an RO/DI water filter to ensure a totally pure freshwater source for making up your saltwater and that one single fish will be the new tank's first occupant. It also assumes that your tank does not have a partial bio filter as is possible with using live rock and live sand. This is for just a basic start up using clean base rocks and a non-living or dry substrate (dead sand). I will explain this process in a step by step manner not so much for your understanding, but to keep my own thoughts in order as well.
1. Research and understand the needs of the ONE fish that you will start out with.2. Purchase a suitably sized tank (10 gallon is a good size) and set it up as a Quarantine tank and place your first new fish within it. This fish will remain in the quarantine tank for a period of 6 weeks while using the Hypo salinity method to ensure the fish is pathogen free. It is important that you do this for every future fish also.
3. After the quarantine tank has been running with the one fish in it for two weeks, then set up your show tank completely. The reason to not set it up earlier is that we want the tank to start a bio filter and be ready at the same time that the fish is ready to come out of the quarantine tank four weeks later. This is important since the show tank will need that fish to keep the bio filter from dieing back and allow it to grow to the proper size according to the bioload of the one fish without producing alot of unneccesary nitrates. If the show tank takes a bit longer it will do the fish no harm to stay in the quarantine tank longer also.
4. Now that the show tank is up and running, we need to introduce an ammonia source, this is up to you which you prefer to use, I would personally prefer ammonia drops since you dont have to remove a stinking chunk of fish meat later, and the drops give you a bit more control on just how much ammonia is being added instead of waiting for something to rot.5. Ammonia is in the tank, now we wait with the tank's lights off for a week and then start doing daily nitrate tests. At this point I am not concerned about reading ammonia levels since we are not going to wait until all the ammonia is converted to nitrates.
7. While you were waiting, it would have been a good idea to research your next fish addition as to it being compatible with your first fish. Once done so, you can put the second fish into the quarantine tank right after the first fish came out and leave the second fish within the quarantine tank for the next six weeks also. This will allow the new bio filter time to grow to the first fishes ammonia output and ensure that no Ich parasites get introduced with the second fish. From this point on, you can repeat this step with each additional fish or coral each six week period until you have your aquarium at proper stocking levels.
8. It is important that during your tank's first two months that you regularly test it for ammonia and nitrate levels. If you can detect slight ammonia levels, that is not a concern as the bio filter will grow and take care of it. Its also very important to keep control of the ammonia levels by not over feeding the fish, one feeding very lightly each day is more than enough. Skipping one day a week is also a good idea. If at any time you see nitrate levels go above "5", then you will need to do a water change to knock it back down.
9. When you started this new tank. You should also have started your nitrate control methods. Over the course of the first few months, these methods will mature and take over your job of controlling the nitrate levels and you will be able to cut down on the frequency of water changes. By using this method you will accomplish: - A gentle, more natural cycle without creating an Algae nightmare and allowing the tank to mature at a proper pace. By the time your tank is fully stocked, you will have a healthy, mature tank capable of maintaining its own water quality and just the joy of having avoided a nitrate tank from hell.
Yes, you can cycle a tank with ONE fish, but not in the manner that it is normally done. Here is how.
1. Follow the same procedures as outlined above, except start the show tank at the same time you start the quarantine tank with its first fish.
2. Put ONE drop of ammonia into the show tank and leave all tank lights off.
3. After the fish has completed its six weeks of quarantine, put it into the show tank and start your second fish in the quarantine tank. Turn the lights on the show tank now.
This may just be an even gentler way of doing a cycle, since the one single drop of ammonia will have created a small bio filter during its four week wait for a fish, and will also not produce many nitrates. Just prior to adding the first fish to the show tank, I would do a large water change to flush out any remaining ammonia and nitrates. The addition of the one fish will then become the ammonia producer which will allow the bio filter to grow only as large as needed to deal with one fish, thus preventing the large nitrate levels we are trying to avoid. Any additional fish if added one per six week period will increase the bioload and allow the bio filter to grow again to the correct size for handling each additional fish, which should be added only after a quarantine period. The same quarantine procedures should be used for all live sand, live rock, inverts and corals with the exception of keeping the salinity within the quarantine tank the same as the show tanks level, Hypo salinity can only be used on fish. Any Non-Fish livestock / Items must remain in normal salinity and in a quarantine tank for six weeks just as the fish do.
4. You should start all nitrate control methods when you set up the show tank, this will further reduce any produced nitrates and may limit the frequency of water changes to keep nitrate levels at 5 or below.
5. This is basically the same method we use normally for starting a tank's initial cycle with one important difference. Limiting / Reducing the initial amount of ammonia which then limits the amount of problem nitrates produced.