Tara Duncan is a French cartoon show (produced 2010, being dubbed for English for presentation on cable TV network Kabillion). It is based on characters from a series of novels where a trio of magical teens encounter various creatures (e.g., vampires, werewolves, harpies, sirens, etc.) [The novels are set on a magical world; the cartoon show is set on Earth.] The trio's mentor is a dragon.
At least three episodes contain merfolk:
Episode 1 (which makes it easy to find), wherein someone/something is stealing voices from people . . .