Boiling will do zero to remove copper. Copper is notorious for remaining for a long, long, LONG time in tanks and can lead to invert wipe-outs years after it was last used in a tank.
I would toss anything that was in the tank when the copper was used. Rinse the empty tank a bunch of times with water. Then several rinses of the tank with water containing a 10x dose of Prime (which can bind copper and other heavy metals). And then cross your fingers, because there are still no guarantees.
(Note: this is what I would do for freshwater invert keeping. If saltwater inverts are more resistant to copper, then what I wrote above might be overkill. I've never had a salt tank so I can't advise you about that specifically.)