Remove all fish or other animals, plants, moveable rocks and decorations from the pond. Fish and plants should be placed into prepared temporary holding tanks filled with treated water (treat the water as you would treat the water in your pond for fish and plants).
Locate the crack. Examine the pond with your eyes to determine whether you can see the crack. If the pond has algae on the sides, use a wire brush to scrape the pond's walls. This should fill any crack with algae, making it easy to see. Turn off the pond pump and use a medicine dropper to drop two drops of whole milk every three inches around the entire perimeter of the pond. You should be able to see the milk be drawn into the crack. Finally, if you are still unable to locate the crack, turn off the pond pump and allow the pond to sit until the water level stops dropping. The crack will be located above the water line.
Drain the pond to expose the crack and prepare the pond for repair.
Use a hammer and chisel to remove any loose debris or concrete from the crack. Chisel away concrete 1 to 2 inches around the entire perimeter of the crack, as well, to allow a large area with which to work.
Apply a concrete bonding agent around the outside of the crack.
Fill the damaged area with concrete patching compound using a caulking gun.
Press concrete repair caulk into the crack with your dampened fingers, scraping away excess caulk with a putty knife.
Wear protective clothing such as gloves and goggles. Fill a plastic bucket two-thirds of the way with water. Fill the bucket the rest of the way with muriatic acid. Carefully pour this mixture over the surfaces of the drained pond to prepare it for painting. Neutralize the acid once the entire pond has been treated by sprinkling baking soda over the acid. Allow the surfaces to dry thoroughly for up to eight hours then sweep any baking soda residue from the pond.
Apply an epoxy-based paint to the dried pond to seal the repaired crack as well as any tiny hairline cracks that may have formed on the pond. Allow the paint to dry thoroughly.
Refill the pond with water once the paint has dried. Treat the water as you normally would for fish and plants. Replace any rocks and decor to the pond. Replace any fish and plants to the pond once the water is treated and stabilized.