I was smart?
It is a Jewish belief (some, not all) that glass absorbs the flavors of food, that it is porous like china because they are made similar. When keeping Kosher we don't mix dairy foods and meat foods. Two different sets of plates are used in the house.
The fellow at the LFS took the belief to a logical end and thought that if food absorbs than the glass of fish tanks will as well.
At first I was angry until I realized he was a very religious Jew, not trying to rip me off.
I don't think he should be laughed at, just not believed in that instance.