I take it this is a pretty new tank? Even after the water levels settle when its cycled, fish can still die later on as they are weakened in the long term. Take heart, after a year things do seem to even off, and new fish that haven't been exposed to poor water quality do better.
An isolation set-up removes a lot of stress when handling sick fish. I just use a clear plastic container with spare filter and heater.Some spare coverage plants (that I often chuck out afterwards, just cheapy stuff). No gravel! I like a seperate set of nets and syphons for my isolation tank to avoid cross-contamination. I keep lots of Prime and zeolite handy as I have better success controlling ammonia spikes in a medicated isolation tank that way.
Isolation tank goes under the desk where it's quiet and out of the way.
Sadly 90% of time when the fish gets isolated, that's pretty much it for them. Unless you have textbook symptoms, it can be hard deducing the best treatment.
