I have to say it's nice to know I'm not the only one who has had this issue. I see all these people on the duck forum with cement ponds and kept wondering how I went wrong.We did a concrete pond a few years ago and also had real leaking problems. I had to patch it and then coat it with a epoxy coating that was safe for fish before it stopped leaking. Then I had to deal with the air bubbles that formed over the years in the epoxy. I would definately use a liner material before using concrete again. Ended up spending a lot more with the concrete in the end (between the concrete, expoxy and the lost water.) Water is scarce in NM and I was losing a lot, but not right at first.