I've read elsewhere/been told by OFL, that coppersafe is some pretty nasty stuff. It's actually a heavy metal, and poisonous to both fish and parasites...it just happens to kill the parasites faster than the fish.
From what I *vaguely* understand about the method of action is that they call it copper*safe* because it's chelated copper...so less hazardous for the fish. But, from what I read, that chelation only lasts for so long, and fairly quickly the coppersafe turns back into regular copper, which is definitely toxic...not immediately, but over a period of time, and even short exposure can have negative effects later.
It's like if you had lead water pipes. The lead wouldn't kill you in a day, a week, or even a year. But you'd probably start having some nasty GI side effects, possible brain damage, and eventually, if you never got new non-leaded pipes, you'd end up dying from the lead in your drinking water. (another toxic heavy metal)
Apparently, back in the 80's, copper (just sheets of it, in any form) was dunked into tanks and ponds like mad because everyone realized it would kill so many nasty things that fish had issues with and was heralded as a wonder-med for fishkeeping...Until someone finally someone realized that it was also shortening the lives of their fish...and NEVER came out of the pond/tank. It gets into the silicone seals, and can even infiltrate decorations and plastic/silk plants. So once something has been dosed in coppersafe, it's got some residual copper in it forever, and can probably never house any invertebrates (like snails) ever again safely.
That having been said, I only use it as an absolute last resort. This is actually the first time I've used it since I found out about the nastiness, and I only left them in there for a short period of time. I don't know if the short-term will work, but I thought I'd try it. I figure I won't know for sure until a few weeks have gone by, to make sure that I've fought off the entire lifespan of the flukes, and not just a portion of it. :) At this point, I'd tried everything else on them, and they were having nasty side-effects from the "safer" meds...and I KNOW coppersafe is *very* effective against nematodes (flukes and other exterior worms)...so I figured I didn't have anything else to lose by trying small doses in QT tanks for short periods of time.
I think if it was me, and they're clear of the slime and velvet, I'd take them out of the copper and just make sure it doesn't reappear. :)
I hope this information helps!! :)