I would go with 20 nassarius [true sand detrivores] and 10 ceriths. The thing you have to watch with ceriths is that they like to climb up on the rim of your tank during "high tide" then they die up there as water in the tank evaps. You need to keep an eye on that or be sure that your water level always stays well below tank rim. Also, because they are always climbing up the tank glass, when they fall they need to be turned back over as they usually can't do it on their own....thus death if not turned back over.
Conch died? Have any crabs in the tank? Crabs will prey on snails, which is why I won't keep any. If you have crabs, find a new home for them as they don't serve much purpose in a reef tank anyway.
How is your bristlewom population? Have any predators of bristles? These, too, are great sandbed detrivores that you want to cultivate.
Do you have cyano?
The best test that I have found to test water circulation on the sandbed is to use a turkey baster and shoot some food very near the sandbed. If the food gets taken up by the current....then you're good to go. If not, then......