I see a lot of posts throughout the forum asking what is cycling so this post is for our newer members that have taken the leap into a great hobby and a rewarding one at that. First off I would personally like to say congratulations and thank you for taking the time to talk to members that have years of experience and secondly for doing your research.

So first off what is cycling?
Cycling is the development of bacteria on filter media, glass, decorations, and in the gravel bed. There are two types of bacteria which are needed to successfully cycle your tank. Once these two types of bacteria are developed your tank is effectively capable of holding a specific bioload (amount of fish waste).

How do I know I am cycled?
Once you have begun your initial cycle you will see an ammonia spike. If you are fishless cycling which you should be the ammonia you add will cause the spike. As the nitrifying bacteria are grown you will notice a slight increase in your nitrite levels. As the nitrite builds so will the bacteria that break down the nitrites. After you see your initial nitrite spike your readings on ammonia and nitrite should drop to zero while your nitrates increase substantially as your bacteria starts and ends your cycle. For how to fishless cycle, read the how to section. There is a great article in there by floored I believe.

How does the cycle works?
The chemistry behind the water cycling is this. Oxygen mixes with the growing bacteria and uses the ammonia and nitrites to feed the bacteria and off gas waste.

The first part of the cycle is oxygen mixing with ammonia and feeding the bacteria. The bacteria in turn off gas waste which is nitrites. These bacteria are called Nitrosomonas.
Here is the chemical development of it:

2 NH3 + 3 O2 è 2 HNO2 + 2 H2O

This shows the ammonia and oxygen mixing and developing nitrites.

The second bacteria present are the Nitrobacter or nitrifying bacteria. Both of the bacteria are beneficial bacteria and need O2 to thrive and survive. This is why when fishless cycling it is always better to use an airstone to provide adequate aeration.

2 HNO2 + O2 è 2 HNO3

This is showing the breakdown of nitrites into nitrates. Nitrates are then effectively depleted by plants that use the waste as food or in an unplanted tank water changes.

What happens if I don’t properly cycle my tank?
New Tank Syndrome is the effect of an uncycled tank. The ammonia levels climb much faster than the bacteria can develop and in turn end up causing death to fish by ammonia or nitrite poisoning.

How can I speed up the process of the nitrogen cycle?
Using old filter media from pre-established tanks or pre-establishing a filter on another established tank will increase the nitrogen cycle speed tremendously. You may also use gravel from an established tank as a seeder. The more bacteria you seed in the tank the faster the cycle will go. For example a tank that is brand new with no seeding may take 6 weeks to cycle. A tank with a pre-established filter cycling may take 3 weeks. A tank with pre-established filtration, media, and gravel may take as little as 3-4 days to properly cycle.

Other Aquarium Articles

Aquarium Cycling - A guide on how to cycle your aquarium.
Aquarium Lighting - Learn the basics of aquarium lighting
Acclimating Fish - A guide on how to acclimate fish.
Breeding Fish - Overview of How To Breed Tropical Fish
Beginner fish - Great Beginner Fish for the Beginner Aquarist
Bad beginner fish - Horrible Beginner Fish for the Beginner Aquarist
The beginners tank - A guide to your fist aquarium
Cleaning your filter - Basic filter management
Carbon and aquariums - Information on the effects of using carbon in your aquarium.
Don’t trust the staff in the fish store! - A text about why you shouldn't alway trust your fish store.
Dry and Rainy Seasons in the Tank - A detailed account of how to simulate seasons in aquariums
Electrical Safety and Aquariums - How to prevent electrical accidents.
Filtration pro and cons - thoughts about filtration
Fry Growth rate - Thoughts on how to optimize growth in fry.
Fish keeping for beginners - An introduction for beginners.
Aquarium Filtration- Llists and describes the different methods of aquarium filtration, and then discusses their benefits and drawbacks.
Finding the Triggers - Information about how to find breeding triggers in different fihs species.
Fry Savers - Information about fry savers and how to construct fry savers.
How to Buy Healthy Fish - A gew tips on choosing healthy fish.
Making a Pebble Cave - A guide on how to make your own pebble cave for your fish.
Modifying Water Chemistry - A technical article about modifying hardness, alkalinity and pH.
Optimizing Growth of Fishes - The factors affecting fish growth - some may be manipulated, others not.
Setting up a tropical freshwater aquarium - Detailed guide on how to setup a freshwater aquarium..
Reverse Osmosis and aquariums - RO is a way to get very pure water in which fish such as dicus and neon tetras can breed and raise their fry.
Safe Rock and Wood Aquascaping - How to make sure that the materials you use in your aquascaping are aquarium safe.
Selecting Community Tank Fish - A guide that will help you choose the right fish for you based on tank size and experience level.
Setting Up A New Aquarium - Help beginners setup their first aquarium
Slime coat - Protecting your fishes vital slime coat
Snails in Aquariums - The biology of snails. Their advantages and disadvantages in an aquarium. How to keep them in check.
Amazon Biotope Aquarium - How to set-up, stock and maintain an aquarium that replicates the Amazon watershed.
Wall mounted aquariums - Information about Wall mounted aquariums
Water chemistry in the aquarium - Hardness, alkalinity and pH are discussed technically, from a chemistry point of view.
Water Chillers - Introduction to water chillers.