do not use fine sand. a shark could inhale it and die, use crushed coral or something smooth yet something that they can't breathe in. what u could do is have mostly sand, except for a thin layer of crushed coral on top of the sand. this way a shark cannot inhale it. also use river stone and rainbow rock as relieve spots amongst the CC to prevent belly scratches. if u use JUST riverstone or rainbow rock, the tank would have a very un-natural look. i have a 125 gallon marble shark tank run by a fluval 104; 105; and 304, 2 box/corner/bottom (whatever u call it) filters, 1 shark internal filter, 1 internal filter of some other kind, 17 clams as filter feeders, and an algae wall. i never do any water changes and have no protein skimmer.