Read up on DSB
's. Less than 4 inches is a SSB
(shallow) and nitrification will occur. That turns nitrite into nitrate. Nitrates in small doses (under 40ppm) are ok for fish but not for corals.
of 6" or more allows for denitrification, the conversion of nitrate into nitrogen gas. This can be seen as occasional bubbles rising from the sand bed.
Now as for leaving the CC
substrate, that would not work because the cc
will work it's way to the surface creating a barrier to sand sifters, etc.
Check out Dr. Ron Shimek's article ondeep sand beds