What should I do?
 

Some kind of wierd disease wiped out my tank including hardy clown fish 7 months ago. Fish donnot exibit any symptoms yet dropped like flies. There is a unicorn fish that seem to be immune to the disease and is the only one left for the last 6 months. There were no head shake or gasping nor did any fish show any single spec of white spot.

My big question is how do I proceed to add new fish? The unicorn fish is fat and happy. Eating aggressively even though it is the only fish in the 1000GL system. I have a holding tank with 6 new fish and they are there for the last 7 months too but I did not dare to add them to the main system.

icliao,

First...what are your tank parameters?

The fish did not die because of poor water quality.
I have two more tanks everything is fine. I am pretty sure it is some strange disease.

My 100GL holding tank has two big Naso, powder blue, purple tang and a powder brown and I was able to maintain it without a skimmer and the fish are fat and happy and without any spots for over 6 months.

I know my sure bet is to remove the unicornfish and run the tank fallow for 4 months. I am wondering if that is necessary or should I add a test fish after quarantine it say a hippo tang and see if it lives.

Wow. There are so many variables in this. So the unicorn fish is fine? No spots seen on the fish?

Just a suggestion: I would also recommend that you move your discussion over to the Marine Fish forums. You'd probably get a better response. You're in the New to the Hobby forum. Me thinks you've been into saltwater fish husbandry for awhile.

I lost all of my fish in the display almost the same time 7 months ago. The fish would be feeding fine and then drop dead the next day. The unicorn fish just seem unaffected and has been the only fish for the last 6 month in display. I suspect marine velvet and the unicorn fish is the only one that has developed immunity against it. It has been over 6 months now and I am not sure if the disease is still in there. I have 5 fish in holding tank and I do not dare to add them and they have been in holding for 6 months now. The fish include a powder blue tang and all healthy and fat.

Thank you. Understood.