Fish adsorb/ absorb (which one is it Byron) water through their skin via osmosis. This happens very quickly, within minutes. Adding tank water to a bag of fish allows the fish to absorb some of the tank water into their body and to get use to the conditions of the new tank water.
Biologically this is not the case. Fish cannot "adapt" this quickly. The less the difference, the less difficulty for the fish, but the greater the difference the more difficult and stressful for the fish. This is in reality an aspect of evolution. The video Mike posted does correctly address this.