I am sympathetic to your loss and will do all I can to help get to the bottom of this. Fist I will address the issue of using copper based remedies with live rock, it is possible to do so depending on the situation. If you consider live rock purely decorative and a natural biological filter enhancement (simply a media to harbor beneficial bacteria), then it is safe to use copper. Understand that certain life forms on and in the rock will not tolerate it (invertebrates like tube worms, crustaceans, etc.). If you are using live rock as the base for a reef and do not wish to harm any life on or in the rock, then dosing your main tank with Cupramine (or any copper based remedy) is not advised. Since Cupramine was already dosed, I will continue.
If dosing Cupramine per the instructions (2 drops per gallon of water), you should have reached a copper concentration of .25 mg/l in your aquarium on the first day. The instructions then say to repeat that dosage again 48 hours later which would have added another .25 mg/l for a total copper concentration of .50 mg/l. Cupramine is still generally effective as low as .2 mg/l and still tolerated up to .8 mg/l. We recommend .5 which is the middle of this range in case under-dosed (should still be at least .2 mg/l) or over-dosed (should not be .8 mg/l). You mentioned that you have some live rock in your aquarium which will displace water. Do you have a deep sand bed or anything else that will displace a considerable amount of water? Or is there anything adding to your water volume (sump style filter or other)? I assume based on your post that there was not an excessive amount of Cupramine in the water as you only administered the first dose which would have resulted in a copper concentration of around .25 mg/l. You also mentioned that the store employee said the copper concentration in your aquarium was "off the chart".......I am curious to know what that value is exactly.
Can you respond with the following information so I can further assist, 1) what is the lot # for that bottle of Cupramine (we have lot #'s for all of our products for quality control purposes, it is a 5 digit # stamped in black ink on the label or the bottom of the bottle and it is not the bar code) 2) what was the exact concentration of Cupramine in your aquarium after dosing 3) do you have anything to displace water or to increase water volume as mentioned above 4) was anything else dosed with this product like a dechlorinator, medication, pH buffer or other 5) what were the following values for ammonia, nitrite, nitrate, pH and alkalinity 6) do you feel comfortable with the diagnosis based on your research (was it "ick"). It is quite possible that "ick" was the culprit as it is usually easy to identify and can plague the fish you mentioned (tangs especially). 7) did you witness any other symptoms of disease like red patches, deteriorated fins, etc. I apologize for the lengthy response and if you feel more comfortable discussing this on the phone rather than through this post, please contact me toll free at 888-SEACHEM Mon - Fri 9 - 5 EST.