I use Amquel+ when I top off or do a water change. I add the Amquel before I add the tap water. I do the same thing with the aquariums that I have in the house. Is this pond inside the house? Does it have a filter that can support a few koi? When you start to add fish, only add 1-2 at a time. This will give the filter a chance to to build enough bacteria to take care of the amonia. I would test the water every couple of days at first. Once your pond is established you can test less often. I test once a week, but some people test every day.

Clarke

Good article at koivet.com on constant flow