I think I might be better off using an example here.
According to the sign, Scorpio is a Water element. However, in an older version of this RP I chose to go with Sand/Desert/Egypt as a theme because scorpions are mostly found in desert areas.
In the previous adaptation Aquarius had ice powers, similar to Mercury, only with a bit more blast, bite, and power to it.
So you don't have to follow the element that is assigned to it according to astrology. Do what you think matches best. Be it air, water, ice, etc.