Special-case FW chilling question

I've googled chiller posts for about an hour, but can't find anything
specifically on this topic, mostly because I'm not vainly trying to
use a dorm fridge. :-) If I've missed something, I apologize.

Basivally, I'm trying to use one chiller to cool two tank,
temporarily. I have a large (~280 gallon) freshwater system for
keeping native fish, with a drop-in chiller set to 15.5 C. There are
other tanks in the room that are passively cooled by the ambient air
(about 18 C), but this system has a couple of circulation pumps that
give off enough heat to need additional cooling. I've set up a
temporary holding tank of about 130 gallons next to the chilled tank,
run by a good-sized Iwaki, which is heating the water up to about 20.5
C. I've tried running the return from this pump through 12 feet of
3/4" ID clear flexible tubing (Vinyl? it's whatever they had at Home
Depot.) that's coiled in the sump of the chilled system, but this
hasn't helped, much. I'm wondering what I can do to increase the
efiiciency of heat-transfer here.

I'm guessing I'd need to add a lot more of the 3/4" tube to get much
improvement, as it's fairly think-walled. I have a spare powerhead
(AQ Systems 600) which I could hook up to .5" ID tubing that might
work better in a dedicated cooling circuit. The smaller tubing's
cheaper, anyway, so I could buy more of it- remember, this is a
temporary (6 week) installation. I'm wondering the following:

Could I put a copper coil in this circuit? Will copper corrode or be
otherwise "bad" in a soft-water system at pH 7.2?

Should the powerhead be in the cool tank and drive water through a
coil in the warm tank, or vice-versa? I'm thinking immersing the
powerhead in the cool tank will put the waste heat there, where the
real chiller can pump it off. The only benefit to putting it in the
warm tank might be that I could return it above the water line and get
some additional aeration (it's holding trout, who seem to be real
oxygen hogs) and perhaps a touch of evaporative cooling.

Any thoughts on these questions would be greatly appreciated.

Thanks,
-Duncan