I've never kept them, but since they are true conchs, their diets should be similar to the others in the family. That would mean some proportion of macro- and microalgae, although the preferred species are likely different than Strombus gigas, S. costatus, and S. alatus (otherwise, they wouldnt' have their own niche in the ocean). I don't know if you've tried to find info on the net, but you could probably search for STROMBUS RANINUS DIET or STROMBUS RANINUS FEEDING and at least find some info.
Cheers,
Don