Replace black plastic liner

[Pete I]

Lined packed earth, cob, FP, 6 mo ago, only 4 mil poly, all I could get, cracking, leaking now. Intend to, remove plastic, smooth walls with cement, then plaster over, water-tight, understand acid bath, for pH after, mine too high now. Thoughts.
Pete

[David Waite]

Pete I would drain and clean liner and leave in place. Cover with 3 inches of fiber reinforced mortar and then paint with pool paint. This will stop the  leaks and provide a ph neutral basin for the fishes. A extra would be to add wire reinforcement but not a must have for your setup.

[TCLynx]

Or you could simply compare costs and see if a high quality EPDM firestone pondguard pond liner would be more appropriate for you.

 

I've also heard of people using wax to seal concrete.