I'm a little confused about what you said in your last 2 post. This one you say adding ice worked did not kill your minnows and in the one before you said "When I had my tank in the garage I tried ice. It didn't work. Always killed fish."
My water has just chlorine in it, I called the water treatment plant and they say they do not use chloramine
Sorry - work has been rough and I've been tied up.
The ice idea does actually work but it involved too much hassle.
Too much ice too fast and you shock them.
Ice made straight from tap water has chlorine.
Let me explain.
I started making the ice with treated water.
And only added the ice in small amounts, but had to do it almost constantly.
It worked, but was a royal PITA.
So I stopped.
If I used ice from my freezer's ice maker it had chlorine in it.
Also, if I tossed it into the tank in large quantities...………..
For your copper idea.
Here's something I almost tired
Take the copper tubing and attach it to an in-tank power head (pump).
Run it out of the tank and into a cooler with a bit of the copper tubing coiled inside the cooler.
Then route the tubing out of the cooler and back into the tank.
Keep ice in the cooler to chill the coils.
Run a hose from the drain of the cooler to a floor drain in your basement to accept the ice water as it melts.
Now you can flow the tank water though the tubing and chill it, and it stays isolated from the ice with the chlorine in it.
I have a keg beer tap my FIL gave to me years ago that runs thru coils inside a cooler like this and it works great for chilling the beer prior to coming out of the tap.
Here's a link to a small power head that would work.
You need low flow but enough "head" capacity to push the water up and out of your tank and then thru the coils and back into the tank.
You might want the cooler up off the floor over to the side of the tank so gravity helps it to discharge.
(The pump doesn't need to pump up out of your tank and then up a second time out of the cooler and back to your tank)
https://www.amazon.com/Tiger-Pumps-Submersible-Powerheads-Hydroponics/dp/B01NBKNG4K/ref=sr_1_14?crid=Z8OUMEJYXBA8&keywords=aquarium+powerhead+water+pump&qid=1578755784&sprefix=aquarium+po%2Caps%2C157&sr=8-14