I work in a small business pet shop, and so me, the owner and the other employee compare "idiot customer" stories at the end of the day. I've had someone try to buy an Oscar to keep in a betta bowl.
We had to explain to a man on the phone once that if he had tap water he had to condition it. He kept getting more and more frustrated, and finally said that he was just going to use toilet water, since that wasn't tap water.
......
The latest story was a lady who came in an complained of having "green stuff" growing in her tank. I said, "Oh, it's just algae."
The lady's eyes widened, and she says, "ALLERGIES? Well how do I get it out?"