The only other thing you can try is Metronidazole, which is used to treat internal protozoan infections but I don't think that is the cause. There is a product called API General Cure that contains Praziquantel and General Cure and you could try that, but it might not help.
Tiger barbs are sensitive to chemicals so it is still probably the copper or something else in the water. But without a lab to test the water, it's impossible to tell.
The carbon should have helped but the fish might have been badly affected before you added carbon to the filter.
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Once you have rehomed the barbs, flush the tank out and try a few tetras, just get 4 or 5 cheap tetras and see if they have the same problem. If they do then there is definitely something in the water that is affecting them.
Sorry I couldn't be more help