03-27-2011, 01:52 PM
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The detriments of using "old" water outweigh the benefits. I agree that moving fish between identical water is probably less stressful, but there is no other possible benefit. I just completed rebuilding 5 of my tanks during the past 3 weeks, and in every case the "new" tank was filled with fresh tap water, dechlorinated, and the fish went in within a couple hours. Wood, rock and plants were moved from the existing tank to the new tank with the fish. Ammonia and nitrite remained undetectable, not one fish loss. Within minutes of being netted into the new tank, many of them were acting as if nothing had happened.
The downside of transferring water is that you are moving bad stuff into the new tank--more ammonia, pathogens, pheromones, waste...things that will naturally accumulate on their own in the new tank without having extra added, especially when there is insufficient bacteria to handle any of the ammonia (if you don't seed it somehow). Somewhat akin to never using the water from the bag in which you bring home the fish.
Seeding the new tank with bacteria is positive, and essential if fish are to go in. Hard objects like wood, rocks, decor, substrate, filter media and plants from an established tank will carry nitrifying bacteria with them to the new tank. Keep them wet, don't wash them, and once in the new tank add some ammonia source immediately, like a fish or two, or else the bacteria will die off for lack of "food."
Live plants also work wonders, provided there are enough of them. They assimilate most of the ammonia/ammonium before even the bacteria can get it.