This fully depends on exposure, (concentration vs duration). In general terms it has been shown that the 96-hour LC50 for mollusks and crustaceans is 28–39 µg/l, with sublethal effects occurring at 1–10 µg/l. Is there a specific worry w.r.t. your cuttle?
PS: LC50 is the concentration at which 50% of a given population of (aquatic) species dies, in this case after 96 hours of exposure.
one of our large octopus died in his tank (3000L) with only a small piece of copper wire in it (electrician had been working over the tank) took about 5-6 days and it was awful. He lost his appetite got more and more lethargic , developed blisters/lesions, seemed to lose his ability to coordinate his arms ....it was just nasty.
After this we relined the tank (rhinolinings ---expensive!) and pulled out all the silicon and replaced it, redid all the plumbing, it was a big job.
well, i`m not wanting to torture them to death as you say, but because i test for copper and none is detected, but i also heard that low traces of copper cant be tested, and i`m just wondering if its copper that killed my cuttlefish which is why i`m asking.