No of Clowns in a 6x2x2?

[wasserscheu]

Depending on the supplier of the tab water, there are often corrections done in the additives given to the water, or whatever. Those can vary, if you happen to catch an "abnormal" day, one could bring the entire fish collection in danger. That has happened before.
There are also plenty of gases disolved in fresh water, which often "pearl out" in the tank or even "in the fish" (via gills, the blood is directly exposed to the water and thus directly reacts on changes of water parameters). Many people let their water stand for a night and/or release tabwater under pressure to avoid this.
Fish are simply not built for rough changes. Osmotic shock is in everybody's mind and shows the sensitive interaction of the fish with the surrounding chemistry.