It depends on the type of crab. Many crabs eat snails as part of their natural diet. What type of crab and what type of snail do you plan on keeping together?
I have a red claw crab... I went into my LFS yesterday and say some really tiny snails in one of the tanks they had, i did not like them...they looked like microscopic snails...what kind of snail(s) can I get to go with my crab.. I only have one crab
Some adolescent mystery snails (2 or 3) would work for you. They will stay close to the water level or usually on the glass away from the crabs. I don't know about your crabs and how they react, but the mystery snails are completely docile and do a great job cleaning! (and come in beautiful colors!!)