The Tails are an essential and main part of H2O:Just Add Water and Mako: Island Of Secrets. They are what make the Mermaid and Mermen them. 

Mermaid Tail

A mermaid tail is a light bronze colour and is very long. 

Screen shot 2013-08-04 at 2.47.38 PM

Merman Tail

A Merman Tail is a light Blue Colour and Very Long.

Screen shot 2013-08-04 at 2.48.04 PM

Behind the scenes

A tail is made with a cast around the actor's/actress's legs, and is custom made. After the tails are made, some clay or flexible modelling material is put in the gaps, then is used to build up the shape. Then, the tails get more than 5000 scales put on by hand, and isn't easy. The fins are made using the modelling material, and painted. Method explained by Indiana Evans or "Isabella Hartley" on the Nickelodeon site.