Other Fish in the Ocean

Things have been flowing smoothly since the girls, along with the help of Zac and Cam, saved Mako Island. It's been a few years now since then and the girls had come to terms that their Pod was never coming back for them.
So they've stayed on land with their Aunt Rita, built stronger bonds with each other, made relationships, after all they are the last Mermaids left in this part of the Pacific Ocean, Or Are they?
What happens when Zac goes out for a swim around Mako, and ends up getting lost. It's up to Cam, Nixie, Serena and Lyla to find him. But, what's this? Zac's met OTHER mermaids?