The fish is bloated, has some type of bacterial infection near the gills and the eyes look like pop eye. Possible internal parasites.
This language barrier doesn't help but we can try.
What is your ammonia at and the size of tank? ammonia is 0 (test api) and aquarium size 660l (135*70*70cm)
Have you seen any worms? I have not seen something simple when I got the last datnoides of the 5 died the next day. And now I want to lose and others.
Tankmates? 4datnoides microlepis 10cm-13cm,4 datnoides 5cm,1 panaq L190 15cm,2 geophagus suriamensis,and 6apple snails.
How's it eating and acting? krill and live krill and solomo.
i believe that it should be Daktylogyros or Gyrodaktylos or something similar.
The issue is what I call must have FGM;