"Can you grow pot in that tank?"
"Is a Blue Tetra a schooling fish? Is a Congo Tetra a schooling fish? They are? Ok, give me one of each. They will school together."
"I sent you the wrong shipping address. The shrimp will die. I want my money back."
"This is saltwater, right?"
"Wow! Look at those magnets! How they clean the glass! Wow!" - common reaction to using magnet glass cleaners in public.
"All our fish are wildcaught! Cardinals? Yes we have. They are wildcaught too." - an Asian fish exporter.
"This is BBA. It is a very bad algae. You do not want it. I can not give it to you." - me talking to a guy that wanted BBA for his tank. Minutes later I said "Here it is. Just don't tell anyone I gave that to you."