Aryana

In the Filipino TV show Aryana, the title character's destiny is set in motion before she's even born. Her father Victor unknowingly killed a young mermaid when he threw lit dynamite into the ocean to get it away from a crowd. The young mermaid's mother, Neptuna (Desiree del Valle) vowed revenge, so when Victor and his wife Ofelia had a daughter of their own, Neptuna kidnapped the baby. Neptuna was unable to magically turn baby Aryana into a mermaid, so she made a deal with Ofelia that she can have Aryana back for now, but on Aryana's fourteenth birthday, she will turn into a mermaid and has to be returned to Neptuna.

By this time, the stress of baby Aryana's kidnapping has caused Ofelia and Victor to separate, so Ofelia takes her small family to start afresh somewhere new, not knowing that Neptuna can magically watch their movements. When Aryana turns thirteen (and is played by Ella Cruz) the show starts proper.

The show ran for 189 episodes. In the early portions of the show, Aryana's adventures are mundane: she goes to a new school on scholarship, gets entangled in love shenanigans with boys, and has a rivalry with her half-sister, Megan (Victor's daughter with his new partner, Stella). But little by little Aryana's mermaid curse kicks in with the appearance of scales, the discovery of her talent for swimming, and the mysterious response of water to Aryana's emotional state.

On Aryana's fourteenth birthday (about a fifth into the show) she transforms into a mermaid and starts the next stage of her life, having to hide her condition from outsiders and deal with the physical requirements of being a mermaid. Aryana eventually gains a magical pearl that allows her to temporary gain legs, and though this helps her resume a normal life, "normal" is relative and the weight of her secret threatens all her relationships.

Once Aryana is a mermaid, there are plenty of transformation-related shenanigans and struggles to cope with her condition, but considering the episode count, there's not that many underwater scenes because Aryana can only visit the ocean occasionally, though Ella is a good swimmer and Desiree as Neptuna also looks great. Neptuna and the merfolk community have their own subplot throughout the series as those machinations occasionally affect Aryana, but it's still secondary to all the land-related plot and Aryana's drama with her family and friends.

There are some references to the Hans Christian Andersen story in the show. In an early episode Aryana is the lead of the school's The Little Mermaid play opposite Hubert, and her tail costume understandably upsets Ofelia. A later episode has Aryana rescue Adrian from drowning, and Megan thinks the rescue is entirely hers and takes credit for it, though as a twist on the Andersen version, Aryana doesn't want Adrian to figure out that she's his true rescuer.

Many thanks to Jay for letting me know about this show!