The friendliest place on the web for anyone with an interest in aquariums or fish keeping!
If you have answers, please help by responding to the unanswered posts.
Cerith and nassarius snails are great snails to have. The ceriths are good algae (including diatoms) and detritus eaters that forage rock, glass, and sand. Nassarius snails mostly stay on the sandbed foraging for detritus.
Just keep an eye on your crab and watch him for evil behavior.
The crab looks like a porcelain crab to me, I think I can even see it's feeding fans in the pic, if it is a porcelain crab...it's harmless and a filterfeeder, the claws are for show. I agree with the nassarius ID, the first could be a cerrith, but seems a bit off to me, probably just my eyes going bad, though