Don't beat yourself up over this; we've all been there. Even in the best of conditions with reefers that know exactly what to do, the survival rate of infected fish is not high. It's a rough side to the hobby.I’m a little disappointed with myself about this one.
Remind yourself that you did all you could do. Learn from what you've gone through, and honor her in that learning.
Take a look at this thread. Start with the "bare essentials" list and you should be able to handle any urgent care needed. If you encounter other issues, you should usually have time to pick up whatever you need. Unfortunately, you got hit with a hard one right out the gate. I know how that is; same happened to me (which triggered me to learn everything I now know - so I'm still honoring those first losses).What chemicals or medications should I have on hand at all times to be able to react a whole lot faster?
Was the sick fish ever introduced into the DT with the other fish? If so, then they may also be infected with the same problem and time may be critical for them as well. Keep an eye on them; if need be, don't be afraid with transferring them into the HT. I know it may seem more of a risk (and it may be), but if they are showing any indications of a problem, it's better to get them somewhere you can treat them.


