Your talking about these toats correct?
http://k2forums.com/discussion/855/anyone-build-a-tank-from-an-ibc-please
If its the color white then you'll be fine rinsing it out with hot water and then letting water sit in it for a day or two. Are you planning on buying it from a private party? The companies that use them give them out for free lol. Local places like co cola factories just throw them away after using the syrup inside them. But make sure its white and its good to go. The blue/black ones are used for chemicals from my knowledge.
Do you mind me asking why you would want aged tank water? Chlorine will evp out but if your water has chloramaine it'll stay in there regardless.