Hi Moonplaty,
As Discuslova mentioned, there is not enough info regarding your situation. I'm a Newbee and won't be able to address your specific situation but it would help the pro's here if you could answer the following questions, this way it we can better help your fishes.
Tank size:
pH:
ammonia:
nitrite:
nitrate:
kH:
gH:
tank temp:
Fish Symptoms (include full description including lesion, color, location, fish behavior):
Volume and Frequency of water changes:
Chemical Additives or Media in your tank:
Tank inhabitants:
Recent additions to your tank (living or decoration):
Exposure to chemicals:
Digital photo (include if possible):
How long has tank been running:
Hope this helps!!