crushed coral has very large grain size and lets excess food get trap in the crushed coral. the trap food then producess nitrate, and that's something you realy don't wan't, also it has to be cleaned or bacumed regularly. i recomed removing the crused coral and adding a deep sand bed, some were around 4in. the live sand also helps with filtration and reduction of toxics. i have a 72 tank with 40 pounds of live sand and 40 pounds of crushed. the crushed coral that is up on top produced the green heary algae that i realy hate and the heary algae doesn't go near the sand. if i knew this was going to happen i added sand only. also if you add sand on top of the crush coral the crush coral will find its way up some how.