As part of its ongoing efforts to promote every single Pokémon out there, The Pokémon Company has shared special content dedicated to Goldeen:
Did you know? Its dorsal, pectoral, and tail fins wave elegantly in water. That is why it is known as the Water Dancer. Its dorsal and pectoral fins are strongly developed like muscles. It can swim at a speed of five knots.
Source: The Pokémon Company