first welcome to the forum and sorry that your fish is sick
Unfortunately the info you give us is not much, therefore a few questions:
how big is your tank?
when did you set up the tank?
when did you add your fish?
how many fish do you keep in there?
how is ammonia, nitrite, nitrate, pH, temp?
what kind of filtration and aeration do you have?
how much do you feed and what?
are all fins clamped or only the tail fin?
any other abnormal things you noticed?
can you post a pic?
Just for now I would test the waterquality and do a 20-30% waterchange.
Observe your fish how he/she behaves after you refilled with fresh and treated water