My Koi fish are dying...

Question
My husband and I have a pond in our backyard with Koi fish.  It's about 17 inches deep and about 20-25 feet wide.  When we moved into the house in April there were 10 Koi, ranging in size from 4 inches to 9 inches long.  They have been doing great and even all survived a complete water change over a month ago.  Now they're dying one by one.  We're hard up for rain and it's been so hot and humid that the water in the pond is evaporating.  We've been topping off the pond with the garden hose.  Once a couple died, we checked with the local pet store.  They tested the pond water, and didn't find anything wrong with it.  They stated the water in town has copper in it which is deadly to fish, so we got some stuff that is supposed to make the tap water safe for the fish.  Well, it doesn't seem to be working.  We carefully dumped 3 gallon buckets of water with the liquid from the pet store in it, one by one yesterday.  This morning there were 2 more dead fish.  We're perplexed by this and it's very frustrating!!  We want to save the 2 remaining fish if we can!

Answer
Hi,
COPPER in the water is bad news as it is toxic to fish also koi thrive better if the water is filtered and they really prefere the water to be aprox three foot deep or more.
You maybe need a lot of that stuff that the pet shop advised to clear the copper.
I wouldn't add any more water but would put a pump in the water to make a fountain  to agitate the water which will dissipate the copper and add oxygen at the same time
I hope you can save the remaining koi.
Regards George.