if you were to fill it to the brim, about 13-15 gallons.
More information is needed in order to answer this question. The size of the pond, preferably in square meters.
A gram is the mass of 1 mL of water. Imagine a cube, each side 1 cm in length. Fill up that cube with water. That much water weighs one gram. A rough estimate is that one gram is about the weight of a standard paper clip.
About enough to fill a 1 meter cube
About 18,000 gallons.
Crayfish are very good at getting out, so it's best not to fill a tank to the top.
1 oz of water