Thanks!
Thanks for your help everyone.
I'm glad to hear that the tank can't absorb the copper (in my case). But I did some more searching and I think the river rock has a high possibility of having absorbed quite a bit. Rather than waiting for it all to leech out of the rocks, I took drastic measures. I moved the white clouds to a hospital/quarantine tank and tore apart their regular home. I'm starting their tank over from scratch. *sigh* Yes it will take time for it to be re-established, but I wasn't too keen on it being a very long time with copper slowly releasing from the rocks. That's the beauty of nanos - I may have a lot of labor on this, but at least it isn't unwieldy to handle.
Now, no new creature will ever come into any re-established tank without going through a month of quarantine and treatment. That way I'll never have to treat a regular tank again - other than a treatment tank.
By the way - can the shrimp carry any disease in? I've heard snails can carry all sorts of things (like ich or flukes) even if they don't become affected, the illness can carry on them. Is the same true for other inverts? If yes, I'll quarantine the shrimp for a month too.