To neutralize the cement such that they don't have any harmful effect on your fishes, use this method which ever suits you. A faster method is to use muratic acid. Briskly scrub all surfaces of the pond with this solution using a brush or broom. The cement may bubble some as the lime works free. This will not harm the pond. Mix more solution if needed. Make sure it reaches all surfaces. Rinse the pond thoroughly, drain and refill. Your pond is now ready for your catfish. Or lime the new Concrete pond, u can get lime from plumbers, Or wash with vinegar or palm oil. Then lime.