The Magical Moon Rings are a mermaid tail shaped with gem in the middle. It was the Moon Ring that allowed Nixie, Sirena, and Lyla to get legs. Principal Rita Santos also teaches the girls to use the Moon Ring when she agrees to help them.
It is implied that the Moon Rings need to be placed in shells to function. Improper use of powers or less powers than required is also implied to cause problems, as seen when Lyla and Sirena attempted to stop the rain using the Moon Ring without Nixie, causing snowfall indoors.