well i looked on a salt analysis of natural seawater and the copper level is at .003 mg/l which should be undetectable with any of the test kits available.

.05 is a little high since even if you're treating for ick the safe copper level range for fish is between .02-.03 mg/l anything over that fish don't tend to tolerate or do well.

safe bet to go ahead and run fresh carbon until you can detect zero levels.