In this video, I have a white long-skirted tetra in the tank with a variety of tetra and angelfish. Symptoms that she is nearing the end are:
(1) Weight loss
(2) Staying away from the other fish, often hiding
(3) Not eating with the others and having no appetite
(4) Swimming with the nose up instead of the usual horizontal swimming
(5) The eyes will often look much larger as the fish loses weight
In a case such as this, it's important not to treat the entire tank with antibiotics or other remedies, as you'd be treating all the healthy fish. Treating healthy fish means they will develop an immunity to the remedies and when they need them, they will be ineffective.
Instead, the two choices are: allow nature to run its course or remove the affected fish from the community tank and place her (or him) in a hospital tank. For the average sized fish, this can be as small as one or two gallons. For an angelfish, it should be 10 gallons. This allows you to treat the sick fish with smaller doses.