Shells from the ocean in a freshwater aquarium? Bad idea?

Yes, they dissolve and affect your water pH.

I don’t think I’d do it. Not only the dissolving thing, but they might harbor wee nasties (bacteria, parasites, funky algae, etc.) that could harm. Most shell-type things I see for aquarium use seem to be sealed/coated with something clear. I don’t know what they use specifically, but maybe you could encase them in something first and use the pretties?

I remember doing this as a child and it killed all of our fish.

Glass or porcelain for fresh water.

I’ve done it with sea shells and beach rocks, but I put them in boiling, slightly-salted water for a while first. That kills whatever incompatible organisms might be on the shells.