The following information has been compiled from various sources and is intended to be an informative resource for anyone who is interested in setting up a sump There is no absolute right and wrong in this hobby; opinions and practices may vary.
When a new tank is setup a chemical and biological change must take place before it is safe to add any living creatures to the tank. This is often referred to as 'cycling a tank' and involves the 'nitrogen cycle'. The information below should help you understand exactly what happens during the cycle and give you some tips on what you can do to make sure everything goes as planned.
Section Headings:
The old way vs. the new way
There are generally 2 ways to cycle a tank:
What happens in a new tank?
In a tank with just water and glass not much is going to happen. In fact even when sand and live rock is present not much may happen right away, especially if the live rock is from an existing tank and/or was very 'clean' to begin with. However you want some specific events to take place, namely the nitrogen cycle.
The nitrogen cycle is the process of Ammonia being broken down into Nitrite, Nitrite being broken down in Nitrate, and Nitrate being broken down into Nitrogen gas. The further you move down the cycle the less harmful these chemicals are. Ammonia in very small amounts can do permanent damage to fish and may kill them. The presence of nitrites is another source of stress that may lead to death. Nitrates are not harmful to most tank inhabitants except in high concentrations (40 ppm is pushing the safe limit). Nitrogen gas escapes to the atmosphere and is not harmful to tank inhabitants.
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When does the nitrogen cycle start?
As mentioned above if you have a very 'clean' tank (sand, rock, and no living inhabitants) it may not start or start very slowly. In order to help it along you need to provide a source of ammonia. A popular way to do this is put a (one) piece of raw shrimp from the grocery store into your tank for a day or two - then take it out. The ammonia given off will provide a source of food for nitrifying bacteria. These bacteria are the little 'machines' that process the Ammonia, Nitrite, and Nitrate into safer forms. Different bacteria are responsible for the processing at each stage of the cycle.
These bacteria colonize the surfaces and interiors of rock, sand, and other items in your tank. The term 'live rock' is used because the rock is 'living' with these bacteria (and other organisms) that are beneficial to your tank; they are the biological filtration component.
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How will I know when the nitrogen cycle has started?
You'll need some test kits, Salifert brand are a popular choice, to test for:
How will I know when the cycle is finished?
After detecting a declining ammonia level you need to start testing for Nitrites. You should see a similar trend in rise then decline over the course of a week or two. When you notice a steady decline of nitrites it is time to start testing for the final stage of the cycle, Nitrates. Again you should see a pattern of rise and decline, it may be another couple of weeks, or more, before the nitrate levels drop close to zero.
When it appears that nitrates have reached zero, or close to zero, test ammonia, nitrite, and nitrate again. Ammonia and Nitrite should be completely undetectable.
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The cycle is finished, now what?
Young tanks are not very stable so you should continue to monitor your Ammonia, Nitrite, and Nitrate for a few months to be on the safe side. At this point it is safe to start slowly stocking your tank, however the longer you wait the better for the sake of your inhabitants well being.
Never add more than one fish at a time unless they are very small relative to your tank size. Each time you add a fish or other living creature it will increase the bioload of the tank and the bacteria levels need to increase to consume the added waste. This will take some time so it is best to wait a few weeks between fish additions.
If a fish or other creature dies, and you are sure it is dead, remove it from the tank immediately. The decaying tissue will cause an ammonia 'spike', or a rapid increase in the ammonia level, which may be too much for your bacteria population to process and it can lead to a 'mini cycle' - which is not good.
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Is there anything to watch out for?
As mentioned above a dead organism will lead to increased ammonia levels. If this causes a mini cycle it will wipe out some or all of your bacteria population. This can quickly lead to big problems. Imagine one fish dies and is not removed. The ammonia becomes too high for bacteria to process. The bacteria in turn are killed off leaving no biological filtration in the tank. Waste levels from other inhabitants continue to rise. With limited filtration they become toxic and kill off other organisms such as corals and fish. If nothing is done very quickly this will lead to a 'tank crash' which is essentially the loss of all or most of the life forms within the tank.
Aside from the above adding live rock can cause a mini-cycle in some cases. If the rock being added is uncured then only small amounts should be added at once. No more than 10% of the total weight of existing rock should be a safe number to use (10 lbs of new rock for every 100 lbs of existing rock). This is because die-off from the rock will cause an ammonia spike, if it is limited to a small amount of rock the bacteria will bounce back quickly and increase to consume the added ammonia.
If the worst should happen you must be prepared to do water changes in order to reduce ammonia to an undetectable level. In some cases you may need to do several large (50%) water changes. Never fully drain your tank as you don't want to further stress fish or disturb your sandbed if you have one.
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Additional Reading
Below is an article on About.com on this topic.
The Cycle
Happy Reefing!