I have some crushed coral gravel, and some aragonite sand. Which one is better? or should i mix them? Also, my lfs has some weird sand that is very very small grain. It almost looks soft. The owner of the lfs says he hates it because it catches to much detritus. Any clue what this is? What about that carribean play sand stuff? does it buffer the ph and alk.? What about normal silica playsand? how would I buffer if I used this?