Oyster shells are great for ponds with low PH levels. These crushed oyster shells will naturally lift the PH level in your pond and can be a life saver in the winter months were rain fall is at its peak. They will gradually raise the PH level making it a lot safer than some additives/treatments that raise the PH too quickly. There is no accurate dosage for Oyster shell but it is recommended to add some to your filter box and keep a close eye on the PH levels throughout the next couple of days.
You will need to put the Oyster shell in a fine media bag (like the Velda Mediabag) to stop the shell from leaving your filter into your pond and for ease of removal. Its highly recommended to keep an eye on the PH levels in your pond using a PH Testkit.