If the fish was highly active last night as you describe and dead this morning, and shows no signs of trauma, it probably died of stroke or heart attack. I have not had one that small in a long time and when I did, I don't remember any dying so quickly, but I have had some decent size fish swimming across the pond just die while I watched. So probably nothing you could have done for it or anything that you did cause it.
I would get the test kits and start tracking water parameters. If this is a temporary pond, you will have to go through the cycle on the new digs, so get comfortable with the tools.
As for additions of water with or without binder/treatment, I believe you could do up to about a 10% water change with no harm done, but that is dependent on the amount of organics in the pond. In older systems, there is enough trash that the chlorine is consumed very rapidly, before any damage could happen to the fish, but if the pond is new, with good filtration, you may need to treat. I always treated and recommend treating as there is always that time that the phone rings during the refill and the next thing you know it is much later, the pond is overflowing and the 10% has been way over extended, with fish dead and dying.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard