Shells and broken coral parts are what is used in bags of "crushed coral" for sale at the local LFS for use mostly in marine tanks. But they are also used by freshwater aquarists who want to artificially raise their baseline tapwater pH to run a tank at a higher pH. The reason is that the shells are made of CaCO3 (Calcium Carbonate) and in our freshwater tanks they will gradually dissolve, combining with various acids in the tank and allowing the tank pH to rise. The amount of the rise will be slowed as the pH gets higher and if the shells become covered with debris or bacteria.
Sometimes a few shells in a tank will not make much difference, but you should be aware of the potential problem and be testing if you do it. Its a serious decision that's usually avoided if possible, to alter your baseline tap water pH to something else and then try to maintain it over the years. Its just too easy to eventually slip up and kill your fish with a hardness/pH change that is too fast.
~~waterdrop~~
cheers lol 
