Crushed coral can be used but it can cause your nitrates to go up as the debris will get in and well not be able to get out. A DSB of 4 to 5 inches will create an anaerobic and aerobic levels that will help deal with the nitrates also. Also with sand you can get sand shifting invertebrates that will help keep you sand clean. With crushed coral they can’t get around and do their job. If you like the was crushed coral looks you can add some to your sand but we careful as it can cause pockets of air which isn’t good on your nitrates either. Hope this helped I’m sure someone will chime in with a better answear.