Leaks in a cement pond can be difficult to find because of the porous nature of the cement. If the leak is too small for you to locate, you will probably need to re-seal the entire pond with a sealant. However, if the crack is visible, there are some things you can do to try to seal it and prevent further leaking.
Empty your cement pond and clean the algae off the walls. Find temporary homes for any fish and plants living in your pond.
Remove all loose material around the crack using a chisel. Chisel inside the crack also, to open it into a "V" shape.
Fill the crack with either a waterproof epoxy or waterproof cement using a putty knife. Make sure there are no air bubbles. Scrape the filling flush with the side of the pond and allow it to dry for at least 48 hours.
Apply a pond membrane or waterproof paint over the entire pond using a paintbrush. Follow the manufacturer's directions. If you did not see an obvious crack, you can try this step first to find out if that will stop any small leaks your pond may have.
Fill your pond with water and place your fish and plants back in.