Would need much more information than you provided. Were it me,, I would see that water parameters were all in line = Zero for ammonia and nitrites, and nitrates under 40 ppm with 20 ppm being ideal. Would be nice to know how long the tank has been running with fish, what fishes are in the tank,what size tank, how long has the affected fish been in the tank, what size filter,what is water change schedule,how much water is changed,what chemicals or additives or medications are or have been used, what dechlorinator, what foods are offered and how often. Answers to these questions can sometimes reveal possible contributers to problems and help eliminate other factors as well.
If all water parameters are in order,, and I were to take a wild guess ,, I might wonder if fish could be bloated. In which case I would maybe withold food for a couple days and review what proper foods are for particular fish I was keeping.
Would try and offer a variety off foods suitable for the fish while not overfeeding.
This is all speculation on my part, but with what little info was made available,,it is all anyone can do.