[1] gills [2] swim bladder [3] Weberian ossicles [4] ears [5] heart [6] brain [7] liver [8] stomach and intestine [9] kidney [10] spleen [11] gall bladder
img from:https://www.anglingtimes.co.uk/advice/science/articles/inside-a-fish
I read about fish diseases and found something called “Fish Hole In The Head Disease”or hexamitosis, respectively hexamitiasis.
Hexamitiasis is a parasitic disease that can affect both freshwater and saltwater fish.
It is caused by various species of the protozoan parasite genus Hexamita. These are likely present at low levels in the intestines of many aquarium fish where they do no harm. It is only when the fish is weakened in some way that the parasites can multiply rapidly and spread into the rest of the fish’s body.
In some cases, lesions appear on the head and flanks of the fish, hence its alternative name: hole in the head disease.
How and why the parasites are able to switch from their normal intestinal form into the systemic form that can spread throughout the body is not known.
https:/petcentral.chewy.com/hexamita-fish-hole-in-the-head-disease/
Fish parasitized with Hexamita may not have any clinical signs. However, when parasites are numerous fish may show signs of anorexia, emaciation, weakness, listlessness, pale gills, abdominal distention, fecal casts, a hemorrhagic vent, exophthalmia and/or dark body coloration.”
https://www.adfg.alaska.gov/static/species/disease/pdfs/fishdiseases/hexamita.pdf
Difficult to say if this is the correct diagnosis. I would say that mdm and longfin show the symptoms emphasised in italics