Crabs can eat snails if the crab is very big and the snail is small... happend to me only once... the snail had fallen on its back and couldnt get up

and a large hermit came along and killed it... a couple of days latter it was walking around with a new house on his back.... guess wich one.

but if your snails are a god size and your hermits and of normal size (mine was freakin huge) youll be ok.
Marc.
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20Gal, 45 lbs LR, 65W PC 10 000K + 65W PC 20 000K + 10Gal sump/fuge
Livestock :hermit and snails, Green star polyps, Button polyps, Finger leather, Xenia, Zoanthids, Mushrooms, Yellow polyps, montipora digitata, acropora, ?mistery polyps?, mistery crab, six line wrasse