Deb Taylor
550 Gal biofalls with skimmer rebuilt last fall.
.....So sorry to hear about your fish.
What you have described are typical symptoms of Copper toxicity.
All that you can do at this time is to rip out the Iongen and toss it and do a 50% water change immediately followed by another 50% in a couple of days.The Carbon will not make much of a difference if your pond is relatively clean. What MitchM says is true, however the organics present in your pond are likely staurated with Copper at this point and will absorb no more leaving Copper in suspension to be absorbed by the fish. The remaining fish may or may not recover. If they do, it will likely be a slow recovery.
I wish that you had read my article before you purchased the Iongen. Sadly, manufacturers are still selling this dangerous device knowing full well the dangers. At least my article forced them to change their advertising from "Safe for Fish" to "Can be used with Fish" which still implies that the device is safe, but gets them off the hook as regards product liability.
I urge every member of this Forum to read this article. What has happened to Deb and countless others could have been avoided if people were given the true facts.
Our fish our doing better in the holding tank for now. Is there a reason to do a 2 stage water change versus a full drain and refill?