Unfortunately you've received much more bad advice than just the copper. Your tank being 60L (16g) was vastly overstocked. In a 60L nano system you should be looking at having a few small fish. The Clown Triggerfish alone is much too big for your tank (even if it was a baby it will quickly outgrow the tank). I don't mean to be sounding negative after such a loss, but we need to get you on the right track for your restart or it will happen all over again and put you off having marine fish, and we wouldn't want that!

As Brew12 said, it's likely ammonia killed the fish, although with that many fish in a small tank they would have quickly been overwhelmed by the Ich.
Do you have anywhere else to go for marine supplies and fish? I wouldn't be returning to that marine farm with the advice they've given you so far. :mad: