Magicians of the Deep: The Gates of Atlantis Series, Book 4

Thirteen-year-old Colin knew nothing of the magic world until he was bitten by a mischievous pixie. He meets thirteen-year-old Alleya on a secret website devoted to magical creatures. Before he knows it, Colin finds himself thrust into a whole new world.

Alleya has been hidden away from the rest of Atlantis because she's half-mermaid, half-Atlantian. Her father tries to ease her boredom by getting her a computer and finally allowing her to go to school. She accidentally lets slip a dark secret - the gates to Atlantis are being shut and it's destroying magic. Alleya is exiled to Indiatlantis.

There, she learns mermaids are getting sick because of the lack of magic, and the tunnels are beginning to collapse. Together, Colin and Alleya must find a way to bring back the old magic and fight to save Atlantis.