I would have chimed in on this a few weeks ago, but somehow I missed the thread. (Kick-ass pond by the way

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Copper is toxic to Koi, and although it leaches into water faster at high temperatures it may leach enough over time at low temps to be a health issue. In your case, running a Bakki Shower may negate any problems if you are following the 5-10% daily water changes they recommend.
In any case it would be adviseable to have the water tested for copper on a monthly basis just to be on the safe side.
If your tests begin to show a build-up of copper, you might consider changing the submerged heat exchange coils from copper to Stainless Steel beverage tubing. It is easily coiled, thin walled, and is commonly used for food grade refrigeration service.