Hazards of Keeping Freshwater Fish in a Container that was Used for Adipic Acid
Adipic acid occurs naturally in beets and sugarcane, common foods, so it is not likely to be very toxic to fish if sufficiently diluted by washing. As @DGS and @ghost state, normal cleaning of the container followed by adjusting the pH for the fish should be OK. If you need to use an electric in-tank heater, though, check that it is safe to use in a plastic container.