Mako Mermaids - 1x26

During a trip to Mako Island, Zac gets in touch with the water of the magic Moon Pool in a night of full Moon, gaining a merman tail and the power to manipulate water. Sirena, Nixie and Lyla, three mermaids who guard Mako Island, are sent to mainland by their fellow mermaids to deprive Zac of his powers, otherwise they will be forever banned from the island.