I also uses crushed oyster shell. Have an 720L, ph would drop from 7,5(tapwater)-6 very quick, so i added 3-4 kg of crushed oyster shell to the substrate, and now it is at constant 7. If it goes under betwen waterchanges i add a little more til i get the balance..Adding it to the substrate is fine since the motoros is working in it all day long, so it gets consumed into the water.