My landlord threw away some giant conch shells (!!) and I rescued them from the dumpster.
I scrubbed them - someone had put glitter glue on one! And you'll want to get all of the soap off for sure. Then I would boil them- depending on the type of shell they'll have that inside spiral and boiling would get in there where you can't reach.
For some rocks I found I also soaked them in a diluted bleach solution. Then I rinsed them off and soaked them in a bucket of tank water. The bleach has to be completely gone and I figured doing it in tank water wouldn't hurt the process.
I haven't had any problems after doing this and putting them in the tank.