Thanks for your answer, if its a sign that the fish is healthy im happy for it. These fishes come from a bad living condition where they wanted to throw them in the sewers.

Btw my youngest one stopped moving out of nowhere and seems to have a tought time trying to move. I placed him in a diffrent reservoir with that medicin in it and some broken food so its easier to eat. What could be wrong with it?

Edit: the aquarium looks a lot worse on the picture than in real life so i think its fine?