Pool filter sand will not alter pH or GH at all. Not one bit. sidp99, it sounds to me like you had an aragonite-based sand...which will raise pH and raise GH (indeed, that is exactly what it is supposed to do and what people buy it to do.)
It is always important when choosing a substrate to be sure to choose either an inert or a buffering substrate, depending upon your needs & desires. Pool Filter Sand is an inert substrate, and will leave water chemistry unchanged. (And as jsoong says, since there are no nutrients built into pool filter sand, it is usually required to put some fertilizer root tabs in it to provide plant roots with nutrients they need.)