Hello on this subject I have helped build 2 saltwater concrete ponds. The first was in a fish shop where I was working. The second we build was in a night club in Buckhead. Both worked great the framing was center block walls supported by rebar and back filled with concrete. Next we placed a koi pond liner on the inside of the pond and filled with rock, sand, salt and fish. We did use a custom made wet dry and skimmer. The block wall was covered by wooden post and the 3 windows were of very thick plexi glass. I cant find my pics at this time for the pond inside the night club or I would post them it was a prize to see.