Play sand is fine. As crisp330 notes, it's not very popular because the small grain size can be a nuisance. Pool filter sand has a larger (heavier) grain size, so it does not kick up as easily. Thing is, "wild" sand comes in many grain sizes, while commercial sand is typically sifted for consistent grain size. If you want to create a truly natural look without having to wild-collect, you could try mixing pool filter sand with play sand. Sprinkle some light-colored pea gravel over that and you're halfway to a biotope tank.