Yes, I should have said that. It would have been a lot simpler. Hey, I think I saw a mini Skippy on Skippy's site. I'm gonna go look. I think though it was for a regular but small pond but still may have some thoughts to add to what I've said. I also did a trickle tower in the middle of the tank trying to make sure there was plenty of oxygen getting in there. . It was a huge, maybe a couple hundred gallon metal horse tank, the kind the kids play in during the summer in the pasture.
Pete, my DH, says that as soon as the cement is dry/hard on the surface, it won't dissolve and continues to cure even in the water. You have not put enough cement containing lime in your pond to harm the fish. I looked around on line after Googling "putting fish in concrete pond" and stumbled across an article on concrete/cement and it's history. It said that a lot of what we call concrete or cement today may not actually have lime in it. So if you have a bag left, you could read it's contents to feel better (or worse). When I thought I had found the leak between rocks in my waterfall, I used Quikrete and as soon as I couldn't scratch any away with my fingernail, I turned on the pump, maybe a few hours later. It is still the same today and that was a couple of weeks ago with water running over it all the time. I didn't find anything online about when to add the plants and fish. I would do it within 24 hours.
Remember, you have many many gallons of water and only a tiny bit of cement there. The cement ponds are a different story as there is a high percentage of cement surface so they will sometimes seal it to prevent leaching of the lime or wash it down with muriatic acid or even vinegar to either etch the surface before sealing and also to clean the surface of the lime.
Here's the Mini ME Skippy pictures. Cute version of the big one.
http://www.skippysstuff.com/minime.htm And the way they show, you wouldn't have to put holes in your container at all by using PVC pipe and maybe some bricks to hold the pipe where you want it. Pipe it into the top and down the center. You could use a 5 way fitting in the bottom with pipe going from each end of the bottom of the 5 way and then an elbow on each of them all pointing in a circle. Come to think of it, the 5 ways aren't readily available so a T is fine. So it would be an upflow type and then just let it flow over the top back down into the stock tank. Cute. Or you could use pipe coming down into an X and then down into another X and put arms going out from both the Xs with elbows at all the ends pointing in a circular motion. Over done maybe. LOL Stick with the simple T like the picture shows.