This is a suggestion on several of the walk throughs online. Using either crushed coral/oyster shells and fine grain sand or multi sizes of sand.I used Portland type 2 cement and oyster shells years ago. I slowly swapped it out for real rock but there are still a couple pieces in my system.
You never have to worry about po4 dropping off with oyster shells.
But sand will look much nicer. Maybe mix sand and shells?