Usally you want more info before treating. Other things play into if a fish is ill or not. This is only one sign that something may be wrong. You need to also consider...
Is the fish acting different?
Are the colors not right?
How are the other fish in the tank?
What are you feeding them?
What are the water prams (pH, Ammonia, NitrIte, NitrAte)?
How long has the fish been doing this?
After you think about these kinds of questions then you can decide if treatment is needed or not.