Hey dude,
Welcome to MAAST. Good to have a new guy on the forum.
Let me clarify what Chris mentioned. Stress doesn't cause ich, the stress reduces the fish's ability to fight it. One of the reasons we feed heavy and introduce garlic to stimulate the feeding response is because a well fed fish can fight off the parasite much easier than a fish that is overly stressed.
Your tank would be toast if you introduce copper in it, and ever hope to house coral.
I would let it ride, keep feeding the fish and maintain a happy little aquarium. But if you are indeed going to upgrade, well that's another story all together. As Chris mentioned earlier, that damsel is a pretty dominant creature... and in a little tank he's going to want to establish dominance which will further stress the other inhabitants. Just for the fact that you've three fish in a 12 gallon will be a bit of a stressful environment in itself.
What kind of clown?