I'm a little confused. From what I've read people have advised you to:
1. not put the shells in
2. test the shells first
3. put the shells in
There's a very simple and smart way to decide this. Test the shells. If you test the shells then you will have your answer to #1 and #2. Why even rely on what people tell you if you can test it and find the RIGHT answer for your particular tank?
But, hey, what you decide to do is up to you. Your tank, your shells, your fish.
Roan