Age of Aquariums — Bobbi J.G. Weiss

While cleaning up the school's new aquarium, Sabrina notices some rather unusual underwater inhabitants. Transforming herself into a tiny mermaid, she discovers the lost city of Atlantis...inside the fish tank!

The great shrunken city is home to a mer-race so old it doesn't even remember its own origins. But the Atlanteans are sure that Sabrina is one of the legendary Conch Queens, come to take the city back to the Endless Waters.

Sabrina wants to restore the city to its rightful place on the planet, but how can she do it without exposing her magic powers to Mr. Krafl and her mortal classmates? And what if someone doesn't want the lost city to be found?