If a parent wants to feed a baby fish, wait until the baby is old enough to be able to eat pureed meats, which are a bit drier/stickier than the baby-food veggies or fruits.
This is usually around 10-12 months. Fish have little bones, so are more dangerous. A parent must take each flake of fish and massage
it between his fingers until it is mushy (that is the best way to check for bones.