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Message: [QUOTE="Waterbug, post: 217310, member: 2322"] Using oyster shells as a pH buffer is not a good idea imo. They react very slowly so they're a poor buffer. For example say your KH is 70 ppm and you get a thunderstorm that dumps 1" of rain which is in the 2 to 5 pH range. That could drop pond pH in minutes, a pH crash. The oyster shells will eventually bring the pH back up but over hours or days. So the fish experience a crash and a swing. Crushed oyster shells will bring pH up faster, but still hours so still a crash and a swing. The finer the crush the faster the reaction. If crushed to a fine enough powder the oyster shells will suspend through the water and react almost instantly to maintain pH. Basically what baking soda does. Baking soda, at least the part we care about, is pretty much dissolved oyster shells. Oyster shells can provide some trace minerals to the water that some people think are helpful. But these are normally already present. [/QUOTE]