This is a video of our old DIY Indoor Monster fish pond. It was 7′ x 4′ internal dimensions, and over 400 gallons. The underwater footage is courtesy of an Olympus Stylus Tough camera. The fish shown are Oscar Cichlid, Pike Cichlid, Bichirs, and our Niger Catfish.
It was built with a basic 2×4 frame and lined with 1/2″ plywood. The plywood was lined with 3/4″ solid foam insulation, this was prevent the plywood from damaging the liner. A Firestone 45mil EPDM liner was used for this project, it was heavy and not cheap but well worth it.