The only turtles I've got much experience in keeping over crushed coral are terrapin's and fly river turtle's. Which both eat coral in nature - and prefer a higher pH, so I guess that should have been explained in my original explanation.
I can understand how keeping something like a mata mata in crushed coral would be a huge mistake, and probably kill the animal.
Just make shure you dont pick sharp substrate. Here you get sharp stuff wich is basicly grounded coral and smood one wich is the normal coral sand, second obviously much better
