CC can,and usually does cause a couple of problems down the road.#1,it's large partical size tends to trap uneaten food,and fish waste.This trapped waste then breaks down contributing to higher than normal nitrate readings.There is also the need to Vaccume crushed coral.This will almost allways cloud the tank water.Although this is not a big problem,it is something you will experence with CC...A sand bed however does NOT contribute to nitrates.And in most cases,if you have a sand bed of at least 4"(DSB) it will actually decrease nitrates...There is also no need to vaccume a sand bed....