My pumice substitute is perlite. Poultry grit is crushed oyster shells (at least around here) - the idea being that the chickens get calcium with their grit. Otherwise, they could just eat gravel.
Gravel will work though. There a people in the wet states planting cactus in the clay pellets that were designed to work in hydroponic systems. My daughter has an aquaponics system and she uses gravel. The idea with gravel or clay pellets is that the plants get watered and fertilized often but they never sit in water.
I would be concerned about fine sand as it tends to pack and turn into concrete. If you can keep it from becoming concrete and its working for you, don't fix it if its not broke.
Daisy
