copper will kill the live rock and live sand, and it will also leach copper back into the water your best bet is to clear out the rock and sand and run a hypo salinity treatment. You can put your rock and sand in the 20g, doesn't have to be pretty, just has to fit under water with some flow. A royal pain, but a solution. If you are not familiar with hypo salinity, read up on it in the top threads of this forum.