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Reusing Dirty Tank Water
I see a lot of people posting on forums about using dirty tank water in a new aquarium setup. They mistakenly believe that this helps to kick-start the nitrogen cycle, when in fact it does not. Nitrifying Bacteria do not live in the water column, and dirty tank water is not healthy for any fish. Always use fresh dechlorinated water for all new aquarium setups!
Of course sometimes there can be an exception to the rule. If you have specialty fish that require very specific water parameters (Cichlids etc.), you may want to use water that is already acclimated to them. However, it would still seem best to use clean dechlorinated water and work to get those parameters where you want them before adding any fish.