What are your parameters, how old is your tank and what are you using for lighting? Here are some questions that need to be answered....
Aquarium Parameters:
Aquarium type: Reef, Fish-only, Quarantine tank
Aquarium water volume
Filtration type
Lighting
How long has the aquarium been established?
Digital image of the whole aquarium under white light
Water quality (be sure to indicate what measurement units you are using, as well as the test kit brand)
Temperature
pH:
Salinity / specific gravity
Ammonia
Nitrite
Nitrate
Phosphorus
Copper
Other
In-depth information:
Have you lost any fish to this problem yet? (see below)
Are any invertebrates affected?
Respiration rate of affected fish (in gill beats per minutes, count for 15 seconds and multiply by 4)
Are the affected fish still feeding?
What remedies have you tried so far?
Digital image of the fish with the health issue, taken under white light -
- if needed, indicate by drawing a line around the area in question.
A short video of the fish (linked YouTube videos work well) - this is perhaps the best information that you can supply, as long as the video is clear.