If the tang die from ich in your tank, you have ich in you tank. If the cooperband get stress, ich will appear....
I have had the current reef tank up for 2 years. Lots of fat, healthy fish. Every tang comes down with ich within 3 days of introduction. Even after quarantine.
So, I bought a new 120 gallon. Added all the origional live rock and corals but left the tank fish free for 5 months all of this past summer to let the ich cycle die off. I kept the fish quarantined in a 56 gallon during this time with no signs of disease. The reef tank has Zoas, Palys, Torch corals, Frogspawn, chalices, Duncans, monte caps, acans, gorgonians, favia, moon corals and lots of young frags of sps. I have kept salt water tanks for over 30 years and this is my 1st real problem with ich that does not seem to ever go away.
In August, I slowly added the fish to the new tank, one at a time. being careful to not add any water from the old tank. 2 weeks ago I added a
2" Regal tang, I quarantined him for 4 weeks. He was covered in ich after three days. THIS time, he is eating like a horse. I soak the food for at least 30 minutes before feeding in Garlic. Feeding him frozen, flake and pellet 4-5 times a day and he is still going strong after 2 weeks in the tank and eating like a champ, still covered with ich. He shows no signs of illness other than the spots and I think this one is going to survive. The others all began with rapid breathing after a couple of days and stopped feeding until they eventually died.
No other fish in the tank (Rock beauty, yellowhead jawfish, royal gramma, 2 firefish, 2 pajama cardinal, 2 Talbot's damsel and 2 yellowtail damsel) has shown any spots or signs of ich. It is only the Regal tang. Thus, my question. After the tang is fully free of spots and several weeks have gone by, would a Copperband butterfly be at risk? I really want a yellow tang but, my experince with the regal tang has me looking at a copperband butterfly instead.
I have even considered trying this on my Reef tank:
Kordon Ich Attack for Fresh & Saltwater Aquariums
It says safe with corals and inverts and I have seen several posts of people on this site who have claimed success. The thing is, it says it takes 4-5 weeks for the treatment to clear the tank of ich. It seems to me that is about how long a fish might take to become immune to ich if it survives.