How big is your crushed coral media? A lot of the DSB prophets say the key to success is different sized grains of media in a DSB, if so, I would think your crushed coral is right up that alley. However, your crushed coral has probably been trapping detrius for the life of the tank, I am not sure I would want to simply cover it up with sand...

I am going to shut up and defer to one of the great ones.