The crushed coral cannot be sifted by any of the critters in the tank, its just too big. This will allow detrius to fall through the cracks and accumulate. In turn, nitrates will get out of hand causing a tank crash.3fsh said:It's better than play sand, which will encourage algae blooms and a buildup of ammonia as the water cannot circulate properly through it, and it has absolutely no buffering capabilities. A bag of reef sand is only $2 bucks more than crushed coral... I was just giving the best "cheap" option.
Sand can be sifted by an assortment of animals. Play sand does not encourage algae blooms or buildups of ammonia, i wonder what your source is for this information. I have play sand and have not had any algae or ammonia problems of any kind (there are also a myriad of ppl using it with great success over at RC). Also, crushed coral will have a very very low buffering capacity as well. It only dissolves in small areas where bacterial action lowers the pH. The effect is pretty insignificant.