More details please. Not very much to go on there.
Water parameters. Temperature, pH, ammonia, nitrite, nitrate.
Details of the tank and equipment. Tank size, filtration, how long has it been set up? Planted? Substrate?
What other fish are in tank? How long have you the fish effected?
What is your normal water change schedule? How much, how often? When did you last change water?
Have you changed a routine in anyway recently? A different water conditioner maybe? Any new decorations? Any new fish additions?