I've ran across descriptions of using concrete for building large aquariums and ponds and such.
Usually they wait till it's fully cured, soak it in water, and also soak it in some sort of acid (vinegar, dilute muriatic, etc.), and then do another water soak.
I wouldn't consider untreated concrete safe at all, it's a pretty alkaline substance. A lot also depends on what the source of their aggregates and additives and such are.