I have the 25 gallon tank running most of the time, once for a while, I substitute 5 gallon of water with the waste water from DT when I do the routine DT water change. I thought that is enough to keep it cycled. Apparently, that was only good enough for quarantine 1 fish, but not cycled enough for all my fishes as hospital :-(
you were right in the first place :-( In 2 days, I have lost 4 fishes including the blue tang to be treated who was doing so well in the past 3 years.
A side observation on my DT, without these fishes, it seems out of balance as well. Soft coral doesn't seem to care. Hammer/frogspawn and zoes seems doing better, where bird nest used to grow noticeable fast and now is dying. Is that because bird nest need more nutrients when fishes are in the tank?