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I use organic dolomitic limestone cause it has magnesium in it as well.
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I've used BOTH dolomite limestone and oyster shells with success. Both
raised the alkalinity and hardness of the water.
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Koi-Lo....
Frugal ponding since 1995.
Aquariums since 1952.
My Pond & Aquarium Pages:
http://tinyurl.com/9do58
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