Answer: You can use a standard concrete mix; 1 part cement, 2 parts sand, 4 parts gravel. Once the concrete has cured you'll need to seal it with a plasticized sealer to keep the lime from leaching into the water and changing the pH. It's often less expensive and less labor intensive to dig your hole and simply set a pre-fabricated pond liner. You'll have an instant pond without the maintenance factors that concrete will require.
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