thats why it's sometimes better to not buy certain fish. I love Mandarins. Had one in observation for about three weeks. It was eating Mysis and brine, it was in the tank with a Melanarus and a Sailfin tang. All doing great! OVERNIGHT, the melanarus was at the surface gasping, the mandarin dead under a rock, the Sailfin covered in Velvet. I lost the melanarus within hours. Put the sailfin in Copper and hes almost done with quarantine.
What could I have done differently? Multiple Quarantine tanks? Dose copper from the get go? Can't use copper on Mandarins. But it didn't happen in the display so I'll keep quarantining until I can be 100% sure that the fish from 'Belly Up Imports' are not gonna kill what healthy fish I do have.
Hybrid TTM the mandarin and into a observation tank would have been my move. It is work though!! and salt..
Obviously there is nothing you can do with what happened as everything happened so fast but you saved the Tang and your DT.
I have lost more fish in observation only then treating... but my fish basically come to me a day or 2 after they arrive at the LFS. Fish sell out fast. I often kick myself for trying to do observation and letting the fish settle. However, others will have different experiences.... probably more so if they can get fish that have been at the LFS for a week or two.