I put this simplified guide together to help me when I finally upgrade my tank and thought it best to share it with you all. It is a rip off of Sue's (thank you Sue) excellent and far safer, more detailed guides which can be found 'pinned' here:
http://forums.thinkfish.co.uk/fishtank-filtration-and-cycling/.
If you don't have a working knowledge of the nitrogen cycle, I would recommend that you read these guides instead. What do we mean by a seeded filter? (Who is this guide for)If a portion of used media from an established tank is placed inside the new filter at the beginning of the direction of water flow ('seeding' the filter), the process of cycling a tank can be significantly quicker, easier on the fishkeeper (by reducing the need for water changes) and (more importantly!) less impactful for the fish. This can be achieved by removing up to a third of the media from an existing tank when adding a tank to your collection or when upgrading and shutting down an old tank by moving across all filter media into the new filter for example.
Here's how to do it:
Equipment: The most important purchase for a fish-in cycle is a testing kit. It must include tests for ammonia and nitrite (test strips often exclude ammonia)
Seachem Prime dechlorinator. Like all dechlorinators, this converts ammonia to the much less toxic ammonium, though it will still show up as ammonia in the test. The ammonia eating bacteria can eat ammonium as well as ammonia so using one of these dechlorinators will not harm the cycle. It is also the only dechlorinator that claims to detoxify nitrite though even Seachem don't know how. The effect lasts about 24 hours and will protect the fish to some extent between water changes.
Finally, of course you will need some mature media from an existing, active tank in your collection. If you are returning to fish keeping after a break perhaps, you may wish to obtain some live mature media in order to complete this method, please see here:
http://forums.thinkfish.co.uk/fishtank-filtration-and-cycling/list-of-members-willing-to-donate-mature-media-for-cycling/Method: The key factor in this method of fish-in cycling is to keep the levels of ammonia and nitrite low, this is made significantly easier if less is being produced(!) - see tips below. You should measure the level of ammonia and nitrite at least once a day. Your aim is to stop the ammonia or nitrite reading from getting higher than 0.25ppm by doing water changes to dilute it.
When you find that the level of ammonia has dropped to zero, continue to test for nitrite the water changes must be continued till that too remains at zero. After a period you will notice it takes longer for the nitrite to rise, and you will need to do water changes less often and of a smaller volume.
As the nitrite starts to be processed by the second bacteria, nitrate will start to rise which is less toxic to the fish and is removed via weekly water changes. When the readings for both ammonia and nitrite have been at zero for a week even though you havent needed to do any water changes, the filter will be cycled for the volume of fish currently stocked.
Tips: Moving any substrate, decor or plants into the tank from a mature aquarium may also help cycle the new tank slightly faster as the beneficial bacteria you are looking to grow love on surfaces rather than in the water column.
Keep you initial stocking to a minimum and use only hardy fish if possible which are better able to deal with substandard water quality. Once your tank is cycled, fish must be added in small batches with at least a week between, and only if the ammonia and nitrite readings remain at zero between additions. It is usually safe to add the amount of fish equal to a third of the body mass of the fish you already have.
You may wish to begin cycling a new filter by seeding it as per the description above and running it in a mature tank aforlongside its existing filtration for a period first as this will most likely be less impactful on both tanks provided the fish can deal with the additional flow.
Do not overfeed the fish. Uneaten fish food will decay producing ammonia. The more food which is eaten, the faster the fishes metabolism is and the more ammonia they produce.
If you have a planted tank the plants will also use ammonia which may impact test results. Plants are also likely to keep the fish safer between water changes as a result. However, remove any dead or decaying plant matter as this will also produce ammonia.