I have concrete water tanks, where I grow ornamental fish.
Recently on draining the tanks to clean, I observed that they do not retain the water. The tanks drain over time, but there are no visible leaks noticed.
Concrete is porous. It is not a water tight material. The only way to make it water tight is to treat it either on the inside (where the fish live) or on the outside with some kind of waterproofing material.