Clams and manatees are not fish. For manatees, they're mammals, and Sandy mentioned barbecue sometimes, which means in the surface they also eats cows.
Also saying fish eating fish is cannibalism is like saying man eating.... anything on land is cannibalism. Fish is a very broad term for underwater creatures. And besides, for man we eat unevolved animals like cows. While underwater, the civilized and sapient fish (can talk, have consciousness) probably eats unevolved "animal" fish.
Stephen Hillenburg have degree in marine biology but somehow he just kinda jumps over this and went "it's not actually meat lol". That's very disappointing.