I have searched high and low and researched tons about black blasting sand for substrate. Nobody can for sure show me a 20 40 medium grit. Tractor supply is trying to tell me that 30 60 fine, is equal according to their system. Way off from what I understand. Is 12 40 grit okay? From a article on this site I can see that 12 40 is a course grit. Would it be abrasive or sharp for snails or my pleco?