Think of aquariums, when you dump a new fish in it will be a bit frantic, unless the water conditions are similar to the old conditions, then it might just suffer a bit of stress from the transport. Having that said, I have slaughtered them the day of, the day after, and the day after that, mainly the first three days including the day of stocking are my favorite if it is just a pond, but you can always have different success with each different batch of fish.