I built a 30 foot oval pond of concrete. Has worked great for thirty years.
Three of the koi are still original homeowners. Great place to daydream.
Three flat levels: one-foot for plant pots, two-foot for walking when I do
an annual clean out, and a three-foot level where the fish hang out when a
heron walks in. Also they like the deep in cold weather. Each level is
about equal in area. Make sure all concrete is made with the special pond
cement. Otherwise, you will never be able to keep it filled. Regular
concrete is very porous. I understand that the pond paint for regular
concrete doesn't last but a few years and then you will have to transfer the
fish somehow and drain the pond. I used pond paint in my concrete block
filter and that worked ok. Good luck.