It's hard to be sure about the shell.
IMO, I would not have it in my tank. Also, decorations should not be transferred back and forth from
QT to the main tank. I have a separate set of silk plants that I use in
QT as needed.
So, here's what you could do if you really like the shell: You need a pH test kit. I like the Aquarium Pharmaceuticals kits for ammonia, and nitrate,
GH and
KH. But for pH I like Wardley. I thought the color range was easier to differentiate. Test the pH of tap water that you just poured. Now pour some tap water into a cup and let it sit for 24 hours. This is called "aging". Test the pH again after the aging period.
Get a container big enough for the shell to fit in, fill with water and let age. Put the shell in, and test the pH after 24 hours, and then to be sure, test after a few more days. If the pH does not change after about a week, meaning that minimal mineral dissolving is happening, it should be safe for the tank. To do a double check, you could also test with a
GH/
KH kit.
For the rocks, you could try to test them with the pH method too. Or you could get some muratic acid, available at a place like Home Depot, and pour just a little bit on the rock. If it fizzes, it should not be used in the tank. If it doesn't fizz, the rock is inert and will be ok in the tank. If you got the rock at an
lfs, it should be safe for a tank. If you collected it yourself, you should do a check on it.