You only need to stand the water for 24 hours IF you haven't got any dechlorinator. There is no point in doing it for longer.
DO NOT add anything from someones Else's pond as you can import illnesses, bacteria and bugs.
Before you add plants dip them in potassium permanganate to clean them.
What is important is to add fish to the pond slowly, one or two at a time otherwise your water parameters will go sky high and make the fish ill.
Mortar in the pond makes the water more alkali which can be harmfull to the fish so I would suggest you get water parameter kits to check exactly how much lime has gone into the water column.
Good luck with your pond
