You may have gotten lucky, time will tell. In any case; IMO & IME, a DT should never be dosed with copper----even if it worked for you. The danger to inverts and the danger of ammonia triggered by the die-off of micro-life is obvious. But another main reason to never use Copper with LR & substrate is that it is impossible to monitor copper levels. LR & substrate absorb and release Cu from/into the water column. Copper must be constantly tested to be sure its deadly to ich, but not the fish. this is impossible in a DT. The only place Cu kills ich is in the water column while the ich is in the free swimming stage. If the water doesn't contain a therapeutic level of copper, 100% of the time, treatment can't be effective. I don't know what happened in your tank, but if inverts survived, I strongly suspect that Cu wasn't maintained at a high enough level to wipe out the ich. Ich cysts can take 9+ weeks to release their new generation of free-swimmers, starting the whole life-cycle over again and I'm afraid that you will see this in your tank. Very simply, according to a Chemist at a big University Aquaculture facility; " If copper doesn't kill most inverts (corals, crabs); it isn't at a level that will kill the ich theronts".
well, it did kill my coral..my GSP, mushrooms, candy canes, etc. died within 2-3 days..but that was the risk I was willing to take since I didn't want to risk losing my $500 black tang, among other fish I have in the system.
my goal was/is not to kill/eradicate the ich completely (I don't know if thats possible). My goal was to ensure that my moorish idol, black tang and scribbled angel, fish that seemed more damaged by the ich parasite survived and "got better"..so far, they have made a 360 degree turnaround to how they were the first week of June to now, first week of July.
These fish went from being spotted in white dots and hiding under the rockwork all day to now swimming freely in the water column with no spots whatsoever. With the black tang being solid/complete black, it's clear there there are no visible signs of white spots left..
of course, 4 weeks later, it does NOT mean ich is no longer in my system and certainly does NOT mean the fish in the system no longer have ich (since visible signs doesn't mean its cured)...
INSTEAD, what it does mean is that the fish have made a 360 degree turn for the better activity wise and physically, as they no longer look frail/weak and spotted. Thus, I am not fearful they will succumb to the ich and die like I did 4 weeks ago, when it was clear they only had 3-4 days to live.
If i do get lucky with this tank, it will be the second time. My other 150g FOWLR had copper dosed in the display over a year ago, and its still kicking strong with no new signs of ich. I even threw a couple mushroom rocks in there, new fish and no problems.
Lastly, I do agree that a display should NOT be dosed with copper..but in my case, I had no choice and based on success with my 150g FOWLR last year, my 250g seems to be heading towards the same "good" result..
HT/QT is best way, obviously..