03-07-2011, 05:56 PM
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While there may or may not be actual "harm" depending upon the tanks it is coming from, there certainly would be no benefit.
Think for a moment why you do a water change? It is solely to remove water that is loaded with crud--stuff such as dissolved waste and urine, ammonia, nitrite, nitrates, fish pheremones, plant pheremones (if you have live plants), etc. Fish will be healther without any of these, so introducing them to a new tank is counter-productive. And in some cases it might cause real health issues. Some diseases can be transmitted via water to other fish--even to humans.
Water removed from a fish tank is good for house plants and gardens, nothing else. If you should drink even a little, you can become very ill, seriously in some cases; I mention this only to reiterate that the stuff in the water is bad, not good.