Ellie and the Secret Potion (Mermaid S.O.S. Series #2)

Misty, Ellie, and their fellow mermaids are Sisters of the Sea, going on exciting underwater adventures to help their fellow Merfolk and the sea creatures they meet. When Queen Neptuna sends them to retrieve the magic crystals that give the Merfolk their powers, they find themselves battling the wicked Mantora, who's always making trouble for the mermaids. Misty and her friends must work together to bring the crystals home and make the ocean a better place for all the sea creatures to live.