We need more info, please fill this out;
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):
Also, include filters you have running, and how/how often you do maintenance on them.
Off the top of my head, it's a bacterial issue. The medications available will depend on what country you reside in.