Have you ever had fish that refuse to eat?

Something I don't believe has ever been an issue I have run into is fish not eating. Each time I have fed my fish when they have needed to be fed, they have always eaten it.

This isn't always the case with everyone though and sometimes, you will go through a time when your fish will not eat for some reason.

Have you ever had an issue with a fish or fishes not eating? What did you do?