Hi and welcome to AA!!! We need some more info here- if you check out the stickies in this section, theres a list of questions as well!
What size is this tank?
Is it cycled?
What are your exact water parameters (amm, nitrite, nitrate, ph)?
What type of test are you using (liquid or strips)?
What is your water change schedule (%, how often)?
How long has this fish looked like this & did it appear quickly or gradually? Are there any other symptoms?
Is this fish or any other fish new? If so, did you QT?
What do you feed (brand, type foods)?
Have you tried any treatments yet?
Do you have a QT?
Some more info will help us to help you better!