The poison waves (secrets of the sky, book two)

Kiya and Kinjal have succeeded in their first quest in the magical world of the Kingdom Beyond, but whatever good they achieved might not be enough. Their faithful flying horses Snowy and Raat reappear and whisk them back to the Kingdom to help the water pari--winged merpeople—who are slowly being poisoned by the very water in which they live. It is up to the twins to figure it out and put a stop to it.

Fortunately, Kiya has the know-how to solve this mystery...if only she can put the pieces together before the evil serpent king Sesha knows they've reentered the Kingdom!

EVERYTHING IS CONNECTED TO EVERYTHING