Name That Fish Disease

Fin rot or dropsey, isolate and treat with maracyn 2.

good luck
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Hard to judge from a distance. (2) Loss of appetite is a general symptom of malaise, like hiding or listlessness or clamped fins. (1) Ragged fins, or fin rot as famman says, is often a combination of bacteria and Saprolegnia fungus. Sometimes the fins have been attacked first by minor skin parasites. (3) Loss of equilibrium is a late symptom of systemic infection, affecting the swim bladder or even the central nervous system. It could be bacterial. It could also be viral or an attack of sporidian micro-parasites. The red streaks in fins are bacterial: Aeromonads are often linked to blood marks, but it could be mycobacteria.

Hopefully the only difference between your source water and tank water is the undesirable stuff like nitrates, nitrites, ammonia and mulm. Does the stess coat that is added to water de-chlorinater really help reduce water change stress?

I seem to remember that it doesn't do that, but I can't find the post...too large of a nitrate disparity will shock the heck out of the fish and probably kill it as it's already weak...I'd use some gravel from the main tank and probably half- and half- on the water (b/t tank and tap) unless it's really of poor quality.

Unless nitrAtes are over 50 ppm in main tank, I wouldn't worry about 'nitrAte shock', or in the old days 'old tank syndrone'. Use mostly fresh water.
Yes, use a good portion of the top layer of your gravel in the hopital tank as well as any ornaments and extra plants you might have.
good luck