Oyster Shells have been used for years by Koi Professionals in order to maintain a balanced PH level in the ponds water. This of course helping to keep the water quality at it's very best. Oyster shells are best used to raise the PH level in water and are therefore only recommended to be used in cases of lower PH levels.
Ideally, Oyster Shells will be located in the last bay of the filtration cycle, although anywhere in the pond would suffice.
Don't forget to keep monitoting the PH levels within your water as this is generally overlooked and take appropriate action.