Thought of a couple more:
"My tank is [white] cloudy because of algae. I need a sucker fish to clear it up."
Oh, well, how about this: "I need a sucker fish/scum sucker/sucky fish because ____. We had one of those before and it got HUGE *holds fingers up at about 4-5"* but then it died."
One lady tied a real plant to her fake plant. Why? I do not know but needless to say, the plant died.
My favorite is the "regulars." The customers who come in and loiter, either talking to me and impeding whatever I was doing or talking to the customers I'm serving. I don't mind people sharing, but when they tell people things that are not true, it bothers me. I've gleefully offended them a few times.