I can't argue, copper is known to leach into everything and leach back into the water for a long period of time. It is well known for being a nuance for future plans. Do not rely on it being gone, ever.
It may be safe, but I too would not chance it. if there are any traces, and they will leach, then most certainly it would wind up in your rock as well(it leaches into everything) which would carry it eventually into the main tank, where it would again leach into the water. HERE this would be a very bad thing.
Do what you wish, we only hope the best, and do not take this all as gospel, just a warning as to what can happen. Good luck in either case.
And if you do decide against it, and want a cheap curing vat, get a rubbermaid container or trash can. Probably even cheaper than a used tank.