Pisces was the constellation in which the planet Pesca was located. It was also, according to the Fourth Doctor, the "last sign of the Zodiac". (AUDIO: Doctor Who and the Pescatons)
In 2009, the "Children of Pisces" were the eighth to be brought under the control of the Ancient Lights when Martin Trueman invoked the sign into his circle. Pisces followed Capricorn and preceded Aries. (TV: Secrets of the Stars)