PlanetCatfish.com

I bought friday a arius seemani and he die sunday night. I changed monday it because I had a garantee. But he also die monday night, just after 12 hours. I think , the first time, because it was the treatment of copper salt for ick. I put all the drop per gallon recommanded, but maybe it was preferable that i but half the drops. The second day of the treatment, I pu half of traitment, 5 hours after, the second arius died.

Is it possible that it is the problem ? The second time, my temperature was about 28 degree celsius.....about 80 degree farenheit. I dont understand, my gold nugget did not died....but is it a problem for him also ???

Even half the recommended dosage is too much copper for the catfish. You need to use no more than one third the dosage. It also sounds like you're treating the fish without putting them in a quarantine tank. If you use one, you wouldn't need to expose your gold nugget to the copper unnecessarily.
Shark cats require seawater eventually (plus they grow really big), so it's not a suitable tankmate for a gold nugget.