Hi,
First, it is pretty difficult to accurately diagnose most fish diseases/ailments. With that said, I will try to help...
First, identify the fishes behavior. Does it act normal? Does it flash and dart around...rapid swimming? If yes, it could either have parasites (sometimes appearing as whitish areas on fins) or a type of body fungus.
Does it act sluggish...hiding? Is it eating well? If yes, This could point to a bacterial infection...including fin and tail rot.
With these things in mind, go to a pet store and look at the various medicines and diseases/symptoms they treat. You may have to buy one or two different medicines to treat the main problem...and any secondary infection.
I highly recommend MARDEL medications...they give good info (much of which I used here)...and can be combined fairly safely.
If it is fin and tail rot, you can simply increase the frequency of water changes...increasing the water quality. If you do this, you will see improvements within a week. My wife purchased some "cute" goldfish...but she neglected to closely scrutinize their fins...they were all ragged...from living in dirty, infected water in the pet superstore. We kept them in a large bowl (two gallons) and changed the water every other day...and the fin and tail rot was greatly reduced within two weeks. Don't forget to condition the water and bring it to a temp that is consistent with the water they are currently in...to avoid shock and stress.
In the midst of all this, keep the temp and water quality stable.
Long winded, but help it helps.
BOMBAY