12-23-2009, 08:39 PM
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The nitrifying bacteria that cycle a tank and keep the nitrification cycle working do not live in the water. They colonize solid surfaces, all of them under water--gravel grains, wood, rocks, plant leaves, tank walls and of course filter media. The only way to transfer any of the bacteria to a new tank is to move over some of the solid material--and provided it has been in the existing tank long enough to become colonized with sufficient bacteria.
Using "old" water is actually more harm than good. Not only are there no nitrifying bacteria in it, there are pathogens, ammonia, and whatever else which is not good to introduce to a new tank where there are insufficient bacteria to handle the new ammonia produced by the fish let alone the old stuff too.
If you have live plants in the new tank, they will, if there are several of them, use the ammonia from the fish and prevent a cycle or "new tank syndrome" as it is known. If no live plants, then transfer over bacteria as mentioned above, and put one or two small fish in the tank. Bacteria must have a constant source of ammonia or they will die off within a few hours. And the ammonia produced by one or two small fish will not be enough to harm the fish but will provide "food" for the bacteria to live and multiply.