Fins and Fairyfolk

A young woman falls in love with and falls pregnant by a merman from the past. They make a journey by water overland to sea. His people are in decline and he is captured to force him to mate with others of his kind. The girl and another female mermaid rescue and free him. An older river merman befriends friend of girl in her youth. She goes with them to their underground river city. Antiques on walls. Mermaids lived in cave houses along the underground river. Inland merfolk have black hair and black shining tails like gold fish. Asian features and almond shaped dark eyes.