Need more info than that. Likely some sort of parasitic attack, but it could be a number of issues, as well as several types of parasitic attacks. Also could be bacterial infections or even fungal infections of many types.
You need to read the top thread in the tropical fish diseases, it will show you what you need to put up.
Or I can just past it here, we need you to answer these questions:
1. Size of tank?
2. Water parameters
d. pH, KH and GH?
e. Test kit?
4. FW (fresh water) or BW (brackish)?
5. How long the aquarium has been set up?
6. What fish do you have? How many are in your tank? How big are they? How long have you had them?
7. Were the fish placed under quarantine period (minus the first batch from the point wherein the tank is ready to accommodate the inhabitants)?
8. a. Any live plants? Fake plants?
b. Sand, gravel, barebottom?
c. Rocks, woods, fancy decors? Any hollow decors?
9. a. Filtration?
10. a. Lighting schedule? What lights are used?
b. Any sunlight exposure? How long?
11. a. Water change schedule?
b. Volume of water changed?
c. Well water, tap water, RO water?
d. Water conditioner used?
e. Frequency of gravel/sand (if any) vacuumed?
How often are they fed?
13. a. Any abnormal signs/symptoms?
b. Appearance of poop?
c. Appearance of gills?
14. a. Have you treated your fish ahead of diagnosis?
Read more: http://www.tropicalfishkeeping.com/...orm-read-before-you-post-61135/#ixzz31jejGl4i