The Full Moon-

After they've been turned, the full moon will cause unexpected phenomenal effects on them, which is different every month due to planetary alignments. 

A full moon has very strange effects on the mermaids. It seems that not only does the full moon give them an opposite take on their original personalities, but makes them quite romantic and seductive.(which can be explored later on in the role play when we decide what to do for the romance element) 

These effects only last for as long as the full moon is in the sky. During a full moon, the moon pool on Mako Island sometimes protects the girls from the moon's strange effects. The girls usually don't always remember what happened to them when they were under the influence of the full moon. 

Only certain full moons will upgrade their powers (not every single one. It depends on a lot of special timing with the planets alignments)