A boy named Zac falls into the Moon Pool when the full moon passes over it, turning him into a merman.Based on the trailer, the full moon must also be able to grant natural mermaids legs. This is shown when Nixie says, "We could if we have legs." Then, they are shown standing on the beach with legs.
Special Full MoonsEdit
On special nights the full moon is different and affects the girls in different ways. On the night of a planetary alignment, if any mermaid is in the Moon Pool when the full moon passed over head there powers will become stronger, causing the girls to not only have water but also weather powers. This becomes even stronger when all of the girls put their powers together, they then have complete control over the weather. This can cause huge electrical storms. On other special full moons such as a Lunar Eclipse if any mermaid is in the Moon Pool when the moon falls over head shall have their powers drawn away for 12 hours, perfect for if you have someone who is trying to expose you, such as Dr. Denman is "A Twist In The Tail". When all of the planets align with the moon, which if any mermaid is in the Moon Pool when the moon comes over head they will lose their powers forever, which is what happened to Charlotte Watsford in "The Finale".