Volume Reduction is an ability that all merpeople are shown to have. With this power, merpeople can shrink objects.
The first time this power is shown is in "Siren's Secret". Zac uses this power to improve Carly and Cam's vocal chords, while the mermaids used it to shrink lemosn and Siren's vocal chords.
This power isn't seen being used after "Sirena's Secret". This ability may be a branch of telekinesis, since that ability may also be used to apply pressure around objects by moving the surface of the object inwards and squeezing them.