crushed coral is a killer for fish i'm afraid, cichlids esp mbuna are diggers and all too often crushed coral will get stuck in their gullet=dead fish, it also traps detrius and will never ever look clean.
also fine coral sand will stay suspended in the water an irritate the fishes gills
also the lignt colour of the substrate reflect a lot of light and washes out the fishes colour and spooks them.
i know it would be a pain but you would be much, much better off tanking that lot out, using a sieve to seperate the coral and the sand and then put in a substrate of 75% ordinary fine gravel and 25% coral sand, (the sand will still buffer)
you could always use some of the crushed coral as media in an external filter, because, and this is only my opinion, that is about the only place it should be used
hope that helps
neil