The trouble with symptoms of "vitamin deficiency" is that they literally run the gamut of every possible thing; skin reaction, anorexia, nervous system issues, etc. Years ago, we had our fish diets analyzed by Michigan State University college of veterinary medicine. They found that we were deficient in only vitamin E and Thiamin. Mazuri, an offshoot of Purina found the same thing and they actually makes a vit E/Thiamin fish paste vitamin.Scratching head also here. I can think of 4 things maybe. 1: Watch the foxface. It could be passive aggressively stressing it in some way. 2: What about diet issues over the long term. Give a run through of typical foods/feeding before all this. 3: Being strange but possible. Lack of b vitamins? I don't know in the case of fish but I see your dosing nopox. Ethanol which is in nopox typically causes vitamin b deficiencies in humans maybe the same in fish. Certain foods can cause this too. Maybe even vitamin E 4: Maybe a strange skin reaction to the formate. @Jay Hemdal do you think this to be possible dermatitis? Maybe the red spot in other thread was actually a hemorrhage? Both those and anorexia in fish can be caused from lack of vitamins.
Once a fish gets into a deficient state with vitamins, and goes off feed, how can it be corrected though?
I agree watch the foxface - what may look like it is being a "buddy" it may be a sign of aggression.
I've heard a number of people having issue with Nopox, but they seem to be realted to bacterial blooms and low dissolved oxygen.
Jay