very true, although i dont know if the copper would have cause devistating results such as these this quick. generally if a fish is copper sensitive you will see them move off of food, and then after several weeks die.
if you pulled the water from your reef tank and you do religous water changes like reefers do, then your water should be okay. many lfs s keep their salinity at 1.014, and to get it up to 1.023-25 it should take you several hours. if this part of the process is done too quickly, that can kill a fish. we all own a hydrometer, and it takes less than 10 seconds to read it, so use it. i just use the hight tech thermometer located in my finger to match temperature, and when it is a particularly delicate fish (such as an angler, or seahorses, or a fu man chu, i will also check the pH to make sure they match. acclimation and transport is where most of the industries' fish loss happens, whether it is on the way to the wholesaler, retailer, or into your aquarium at home.
sorry to hear you had such a bad experience, hopefully when you try your next one it will go more smoothly.,