I have a 4ft by 3ft frog pond and water won't stay in it for more the 12 hours.I want to keep it filled without using a nylon or any type of liner.Oh and its about 1 foot deep.
Concrete is a difficult material to use. It's brittle and unforgiving. It leaches out lime, so screws up your pH. And when the ground moves, the concrete cracks.
Try bentonite. It's cheap and easy to use. You line the hole with it - it's a clay and holds water.