It's Uronema! Very common with chromis and damsels nowadays. It kills very fast, like the third day. First day looks great, second the side sore appears, third day dead. So, you really need to have the right meds ready on day one. Sorry for your loss.
For uronema, get Seachem Metroplex and Focus. This gets the medication on outside and inside of the fish, which is needed for uronema. Follow all directions to the letter! The type is very small on the packages, so have a magnifying glass handy.
For prophylactic treatment on first adding fish to QT, I like API General Cure. It has both Metronidazole and Praziquantel in it. Also, over the first week, I gradually lower salinity to hypo salinity levels, to treat for ICK. It's much less dangerous to the fish than copper.
The tricky thing about Uronema is it doesn't require a fish host to survive. It can survive indefinitely on detritus alone, so if you've ever gotten Uronema in your QT, you still have it! You have to bleach sterilize everything wet, after EVERY QT treatment. That goes for your display as well, if the sick fish was in it.
I'm no master of QT. I've been as slack as anyone else. I had to recently up my game in order just to keep some damsels alive. Hope this helps!
Yeah, the prospect of breaking down a display is a huge bummer. I would think you could dip your corals, etcetera in a coral dip.
Bacterial infections in general mainly affect fish with a compromised immune system or injuries. Like the regal angel. It was new and probably kind of stressed/weakened from shipping, and it succumbed to the infection. I think your other fish will be fine.
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The tricky thing about Uronema is it doesn't require a fish host to survive. It can survive indefinitely on detritus alone, so if you've ever gotten Uronema in your QT, you still have it! You have to bleach sterilize everything wet, after EVERY QT treatment. That goes for your display as well, if the sick fish was in it.
I'm no master of QT. I've been as slack as anyone else. I had to recently up my game in order just to keep some damsels alive. Hope this helps!