Prepare a holding tank for your fish by removing enough water from the middle of the pond to fill the tank with 6 to 8 inches of water. Remove the fish and place them in the tank no more than 30 minutes after filling the holding tank. Leave your fish in the holding tank until the pond is finished and ready to hold water and fish once more. If you do not have fish in your pond, proceed to step 2.
Clean the walls and floor of the pond thoroughly. Remove any crumbling stones or rocks and wash down the interior of the pond with soap and water, using a pressure washer if possible. Allow the pond to dry thoroughly after cleaning.
Apply a coat of concrete sealant primer to the pond's interior surfaces, according to the primer's directions. This type of primer leaves behind a rubbery substance that fills cracks and makes the cement watertight. Allow the primer coat to dry, then apply another coat. Allow the last coat to dry completely.
Fill any remaining cracks, holes, rough spots and pond joints or seams with a cement joint compound. Apply the compound directly to any damaged areas of the pond, according to the compound's directions, to form a patch. Allow the compound to dry completely.
Apply a coat of cement sealing top coat to the interior surfaces of the pond, according to the sealant's directions. If the hard interior surfaces of the pond are porous, apply up to three coats of sealant to the the pond. Allow each coat to dry thoroughly before applying the next coat. Allow the last coat to fully dry.
Refill the pond with clean water. Once the pond is full again, float the fish back into the pond by placing them into buckets, trash cans or plastic bags full of the holding tank water. Allow the fish to float in the pond in the containers for 20 minutes, allowing the water temperatures of the pond and containers to equalize. This prevents shock to the fish. Carefully release the fish back into the pond.