My suggestion would be to reconsider the pond liner. I have two friends who tried small concrete ponds but both developed leaks after a few years. They were told that they probably didn't have a proper layer of sand underneath to support the concrete uniformly. Most waterproofing will affect fish.
In a previous house I recycled a large fiberglass form used as a fish box in a boat. I put a reinforcing coat of fiberglass on the outside so it wouldn't affect the fish.
I now have a 3x5 pond made from heavy liner which is still going strong after 5 years with plants, fish, and bull frogs. There are in balance and I don't feed the fish. I have a small solar pump to keep the water moving.
Someone on here may have practical suggestions for a concrete pool. Obviously concrete swimming pools are sturdy but I have found that the liner is much easier and is durable. By digging the hole you have done the hard part.
Good luck with your progress.
