I understand it is traditional for folks to ask for help while providing no information. I will violate “the rule.”
Assuming there are no raised scales
'It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts.'
Sherlock Holmes Quote-A Scandal in Bohemia
, there is no infection as such. It is most likely constipation from dry or pellet food.
Stop feeding dry or pellet food immediately.
Given that it was too much trouble to share information about your fish or tank, I will give general advice and only because no one else has answered.
Canned peas are kind of the traditional laxative in the fish world. Whole cut up or slightly mashed depending on the fish. If you happen to have something like Egeria
densa or Elodea
canadensis par boil, cut or lightly mash up. Spinach thinly cut, zucchini, cucumbers, Romaine, curly kinds of lettuce, cut thinly, perhaps par boiled.
Any high-fiber foods of any sort they will eat, preferably live, but frozen if that is all you have, Daphnia, Copepods, Brine shrimp.
The standard fish laxative dose of Epsom salt runs 1-3-teaspoons per 5-gallons (20-liters) (if you are unsure of actual water volume subtract 15% from tank volume), kind of fish dependent, if you are unsure, dose 1-teaspoon per 5-gallons, something is better than nothing.
It is important to get those fish pooping properly.