Question
my son wants to put a round moot fish pond around a 25 foot tall maple tree in south carolina. to prevent killing the tree what is the closest to the tree can he get and go two feet deep with the pond.

Answer
Do not go any closer than the drip-line of the foliage--the drip-line is the area that the branches would make a shadow IF the sun were directly over head. If you outline the ends of the tree's branches on the ground this is the area to stay out of. The tree's roots are here and a 2 foot "pond" would kill the roots and possiblly the tree.