They will slow dissolve in your tank and raise the ph BUT.... it will be a gradual rise and take a good while for it to happen and if your doing regular weekly water changes they may not raise the ph at all. The reason being you will be removing the tiny bit the shells would be dissolving each time you do a water change. Is that confusing? I hope not

I would probably boil them to kill any bacteria or germs that may be on them. In case there were any pollution where you found them boiling them should clean that off also.
Carol