It's also possible that the bulk of it is coming in from certain types of food (macroalgae perhaps), and in smaller hobbyist sized systems it is building up to higher concentrations than you guys see it in your big exhibits, or you aren't feeding those types of food to begin with...(???)...all just guesses of course.
Some marine planktonic feeders like krill will sometimes have high copper counts, especially with the farm raised for fish food varieties. It varies lot to lot, so you can't really pin it down. Food and environmental additions like exposed corroded copper wire/pipe are more likely to contribute toxic heavy metals than from the salt blend.