i was wondering why/if you have to mix the concrete with caribi-sea sand? in rc there is a thread about using pulverized limestone as a substrate, and people do use linestone in their tanks, so can you make the rock with it to?(if you need to mix it) i guess i don't really see the reason to mix with sand. one more question, can it be any type of sand. here in texas, the good stuff is far and in between.(atleast for me) thanks for the help