When my pond was first dug we would catch bass and crappies from different lakes and put them in the pond to stock it. The idea was that they would reproduce. The reason that I have sunnys is because I dumped some live bait in there that had sunnys as minnows. Unfortunately they also have reproduced and they are stunting the growth of the other fish. It is kind of neat to walk around however. I have a lot of bass and walking around the pond will spook frogs so they jump in. Occasionally they jump right in to become fish food.
...gosh, I am repeating myself.... the cancer fatigue, depleting my energy and recent med changes could have something to do with that but frankly, a little redundancy is the last thing I am worried about.
You can buy reconditioned IBCs, and they will specifically say what is safe to put in them. As discussed, there's a reason you can't put food into a cleaned tank thats had chemicals in it. It's much safer to buy a food grade tank and you can buy second hand drum kegs for about £10. http://www.deltacontainers.com/Drum-Products/Reconditioned-Kegs.aspx