The pool filter sand I use-I got at Home depot Quikrete 50lb bag for under $8.00.
I also buy the 50lb bags of the small diameter pea type gravel from Home Depot or Lowes that I use in my aquarium-its a natural collect rock used for landscaping/gardens-Is inert so it doesn't change water prams-you just want to watch out for dyed rocks and rock that can change water prams...