(A middle aged couple come into the tropical fish section and ask me about their algae-eating fish.)

Customer: "Hello, we bought some algae eaters a while back, and they died after about 2 weeks. We bought some more and they did exactly the same after 2 weeks and we were just wondering what could have been wrong with them?"

Me: "What have you been feeding them?"

Customer: "Nothing, we thought they ate the algae."

Me: "They do, if there’s enough to go around."

Customer: “Oh dear. Well they did keep the glass awfully clear."