Hello John. I don't see why you couldn't add it to your fresh water aquarium. People do add sea shells to their tanks for decoration. It could raise your
pH a bit depending on how large a piece it is. We recommend adding crushed coral for the purpose of raising pH levels. I would think you would be fine to add it. I would suggest that you boil it first though to remove any critters/bacteria that may be on it that you don't want inside your tank.
I'm sure you'll get some more responses as well.
Ken