Chullin 66 - Kosher Fish
Any fish that has a fin and a scale is kosher. If it does not have fins and scales when young, but will develop them later, like sardines, it is also kosher. If it has them when in the water but sheds them when caught, like tuna, it is kosher.
Any fish that has a scale certainly has a fin. If so, why did the Torah list both? - To make it abundantly clear for those who don't know the Torah that well. Furthermore, the Torah said " Those that have fins and scales in the seas and rivers you may eat, and those that don't have the fins and scales in the seas and rivers you may not eat ." Why is this repetition? - To make one who eats fish without fins and scales transgress both a positive and a negative commandment.
Since the Torah only mentioned the requirement of fins and scales for seas and rivers, those worm-like creatures that originate in a vessels are permitted - as long as they have not left the vessel. A practical application is that one may drink water and not worry about worms in it, since they are kosher anyway.
Art: Alexander Adriaenssen - The Fishmonger