That's velvet. The freshwater dip was a very good move as it removes a tremendous number of velvet parasites from the gills where they do the most damage.
The copper only kills the parasites in the "Free swimming stage" once the copper has reached therapeutic levels, so the fish will have to weather the parasites already attached. And those spots are from a mucous response from where the parasites have already attached and or detached. It will take a few days for the spots to reduce in number.
Consider adding an antibiotic to ward off bacterial infections that are secondary to the parasites attack on the fish's immune system. Kanaplex, Furan2 are very good wide spectrum antibiotics.
All the fish in your tank will need to be treated. And the tank will need to go fallow for 76 days.
Marine Velvet Disease (Amyloodinium ocellatum) What You Need To Know: * Fast killing parasite capable of wiping out most of your fish in just a matter of days. Sometimes there are survivors, sometimes not. * Can be treated in a quarantine tank using Chloroquine phosphate or Copper. A 5...
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