Oyster Shells have been used for years by Koi professionals to help with the pH of the water and to maintain optimal water conditions and is extensively used in Japan.
Since the Oyster Shells will generally raise the pH level in your water, these should only be used in cases of low pH.
Oyster Shell generally will raise the level of the pH.
For those that ever ever been to Japan, Oyster shell is used extensively by the breeders in Japan, these soluble (soft) shells are used to stabilise the pH.
Oyster shells stabilise the pH / KH-value which prevents pH crash. If the KH value drops to this starts to dissolve the shells until balance is reached again. Nitrifying bacteria also grows on the oyster shell.