Yiks! I just left for a little bit! SoRRY!
So sorry Danny to hear about Mr. Phoomps. I know you loved that guy. He'll be dearly missed, I'm sure.
*SUPERHUGS*
My speculation of the events....
Since this seemed to all have started during treatment. It is possible you lowered the salinity to fast and without closely monitoring the NH3 spike (how high did it go?), knowing scaleless fish (Mr. Phoomps is one of them) are very sensitive, intolerant of ammonia. And without testing... its possible Mr. Phoomps suffered mild ammonia burn when it spiked (or still), teamed up with the ich infestation (too much) it would left him with a wicked bacteria infection.
Now without knowing your water quality levels, you'll never know for sure.
Now if he was still alive.... It's hard to say what would have been best to do. Treating with an antibacterial medication (for scaleless fish) would have probably been my opinion, that's first IF it didn't go away on its own or clear up with having excellent water quality over about a weeks time. I agree, water quality is key.
Just for your info, its not uncommon after dropping the salinity (Hyposalinity treatment) to approx. 1.016 to get a ammonia spike, if your not watching closely. It can mean you’re dropping it to fast. You need to give enough time for bacteria to balance in your system, to be stable and not crash. Most fish are hardy enough to handle a spike unless their scaleless or heavily infected with a parasite.
Also if you’re testing (Nitrate, Nitrite & Ammonia) regularly then you could have caught the spike before it happened. Cause you would see your Nitrates would be rising and rising. It does happen fast though so you have to be on top of it when you know your passing about 1.016.
Please be comforted to know that scaleless fish are one of the HARDEST fish to treat once they get ill. They are very sensitive to treatment & limited for meds to use on them.
But you have other fish still in QT
...so run HOME...check water parameters, quick! Let us know if the others show an infection of the fins as well.
Deepest regards for Mr. Phoomps. *HUGS*