Hello Kesini. Many ponds are easier to keep with a rubber roofing liner, specifically 45 mil epdm. This protects against leaks, and a cement pond can have difficulty staying at the correct pH for fish. Also, cement might be rough and fish could scratch themselves against it. I see many ponds with sand on the bottom so the liner lies on sand, and if you walk on it you do not tear the liner. It sounds like you have good experience with cement building -- you could build the sides using the rebar as you suggest, then pour sand in the bottom, lay the liner, then pour a nice cement border/surround on top to hide the liner edges, even make it look like natural stone.