nothing. unless your fish was poisonous and your cat needs medical attention there is nothing you can "do" to your cat. what a lucky treat for him! Unfortunately, it's only in their nature to go after something smaller. Next time, make sure you have a cover for your tank so your cat can't access the fish. Wire mesh lids work well for labyrinth breathers like gourami and bettas.
my kitten would lie on top of my tank with her paw dangling in until a curious guppy swam along... so we custom fit a wire mesh back to the tank where the filters and tubes came out. Now my guppy population is stable. but she still looks!
