Because I am aware of all of the variables that affect concrete, my suggestion is a pond liner. But any marine grade epoxy, or even fiberglass resin will seal it, and keep the concrete from leaching unwanted chemicals. A pond liner will allow for the possibility of cracks, which are common in concrete, even, when poured by professionals. If I were seriously thinking about using concrete for a tank, I would purchase something precast. But, honestly there are many other options out there that are more practical. Once you pour a cement pond, you are married to it. Moving one is rather difficult. A large galvanized stock tank painted with the same epoxy, is much more portable, and likely less expensive. I have also built large tanks out of plywood, coated with fiberglass resin, and they have held up very well, though, I would not recommend moving them much, as the resin becomes brittle. The pools, like badflash uses seem to be about the best bang for the buck.
I have been working with these recently http://www.grangerplastics.com/gp1004.html and cannot speak highly enough about them. They are nearly 3/8 inch thick, and cost me $200 each. Topped off they are 360 gallons, and should last a lifetime.