Yeah, if the snail is too large to fit in their mouth and crush,they'll suck the snail out. when I used to keep puffers, I set up a 10 gal tank and went to Walmart for snails. They had pond snails all through their filter system and were thrilled to give them to me. Put them in the 10 gal, gave them lettuce, algae wafers and those seaweed sheets. They reproduced like crazy and I always had dinner for the puffs.