No, they don't contract ich nor does ich use them as hosts however they can still carry the cysts or the free swimming stage ich in their shells and you risk transferring them all around especially when the tanks have fish in them where ich uses them as their hosts. If you could separate the snails from fish for a few days to at least two weeks, ich is unlikely to survive for a long time. Please be aware that any chemicals used to destroy ich can potentially kill your snails especially malachite green as active ingredients so it is advisable to separate the snails when treating. The only ones that would likely survive the treatments are the MTS as these are nearly indestructible and are adept at veiling themselves from toxic chemicals.