Actual Pesach conversation w/ a ‘Balabos’ of mine:
BB: Do I need fish food that’s Kosher for Passover?
BB: Why? A dog wouldn’t eat fish food!
AR: True, but your fish would.
BB: What should I feed my fish, then?
AR: Matzah. Unless you have chassidishe fish that don’t eat gebrokts
BB: Can fish eat matzah?
AR: Why not? They may just be swimming near the bottom of the tank for a week, and you may have to clean it really well after Pesach.
BB: I’m Sefardic, do I can feed them rice, right?
AR: Maybe you should just buy some Kosher for Passover fish food. Or worms.