Aquarius is the 11th sign of zodiac and Pisces is the 12th. Due to precession of the equinoxes the shift of the ages occurs. That is 1 degree every 72 years. So Aquarius is not literally opposite Pisces, it is 30 degrees, or a semi-square. Opposite Pisces is Virgo.
This is a very deep subject. Thank you for brushing upon it.