I use turbo snail shells from my reef system as decoration in my African tank. Just be sure to boil them for 15 minutes or so. To be extra sure I'd boil them, then rinse them out with warm water, then boil them again to be extra sure there was nothing harmful.
Make sure the air is out of the shells too. Submerge them in water them turn the opening up and then roll the shells the opposite direction the spiral runs to let all the air out. When you rinse it you'll be surprised how much junk is deep inside the shell.
As for chemistry, a hundred pounds of substrate and rockwork make a very small impacted on your parameters, a bunch of shells would do even less. Good luck!