africans like their water hard, real hard, so something like that would probably be OK
however, I'm certain I read somewhere that coral sand can contain very sharp pieces, and cichlids like to play in the substrate, and can ingest a piece of this, cutting their mouths or worse!
I'm not sure what the southdown sand is made from - perhaps it is more apporiate?
try asking in the saltwater general forum for information on fine calciumish gravels.