A “cycled aquarium” is a tank that has established bacterial colonies to process fish waste, in the form of ammonia, into nitrates.  It is very important for aquariums with fish to be cycled, as ammonia is toxic to fish.  Adding fish to un-cycled aquaiums will generally result in most or all of the fish dying.

What cycling is NOT

Despite the bad advice of so many pet store employees, cycling does NOT mean adding water to the aquariums and letting it sit and filter for a few days.  Doing that can be useful to make sure your equipment doesn’t die right away, but does nothing to cycle the tank.

How to Cycle a Tank

There are three methods for cycling:

• Fishless cycling
• Fish-in cycling
• Seeding a tank

EVERYONE who ever keeps fish has used one of those methods.  Those that are not aware of the choice have chosen fish-in cycling by default. 

Fishless

Fishless cycling requires adding a source of ammonia to the tank which will let the bacteria colonies build up to process the ammonia.  The most straight forward source of ammonia is from a bottle.  You’ll want to look for “clear ammonia” that does not have soaps or fragrance additives.

There are many, many resources available on the Internet to help with fishless cycling.

Fish-In

Fish-in cycling uses one or more fish as the source of ammonia.  For those who consciously perform fish-in cycling, a single fish is generally used.  Zebra Danios are probably the most common intentional cycling fish, because of their small size, hardy nature, cheap price and ready availability at most any pet store.

Seeding

Seeding a tank is basically transplanting bacteria from an established, cycled tank to an uncycled tank.  Commonly, gravel or filter pads from the established tank are suspended in the new tank.  A commercial product, called Bio Spira, is also avalable from some retailers.  Bio Spira is a pouch containing a pre-cultured bacteria colony ready to go.

Note that there are MANY products available for sale at stores that claim to cycle your tank, or “help the cycle”.  These don’t actually contain bacteria, meaning that you will still be cycling your tank when using them. (that includes the product called “Cycle”).

Recommendation

Fishless cycling is the generally preferred method for cycling tanks now.  Fish-in cycling is considered inhumane and inflicts damage to the gills of the fish and reduces the fish’s life expectancy.

Post from: Tropical Fish Central

Cycling a New Freshwater Tank