I use standard small agate gravel that I get in the concrete dept of the local hardware. Washed, sifted and really natural looking with the limestone "holy rocks." A huge bag is only a few bucks. I use non leaching substrate to keep pH shift to a minimum as I do frequent water changes. The limestone does however raise it and hold it fairly constant.
Have a terriffic day !!
2-30 Gal. fry tanks, 75 Gal. Community Planted,
75 Gal., 110 Gal. & 150 Gal. Malawi