Definite recommendation for pool filter sand as it is much more controlled to specific grain size without the trash fine stuff that can give so much trouble. The color can be vary at times so a trick to telling what is in the bag is to look at what is one the floor around the display!
The point on sand is to not get it too fine so that any fish movement or work like moving a plant gets sand kicked up to get into filters. A bit of sand in a filter can stick between the shaft and impeller and wear them out in a couple days! Filter sand is just the right size for what I do. Doesn't pack too tight and doesn't get kicked up to cause trouble. If too bright white, I cut in small portions of a darker sand.