I have a 5 year old Oranda goldfish that I rescued from an 8 gallon outside pond about 6 weeks ago.  (The owner moved into a nursing home.)  The fish is about 10 - 12 inches long and it's the only fish in a 47 gallon tank.  I've had the water tested at a large pet store and am told the water is fine.  However, I'm certain something is wrong.  About 5 days ago the fish became lathargic, hanging out near one of the corners of the tank with head down.  Some days it eats; other days it will take food but then spits it out.  The vent appears larger than before and the area around the vent appears swollen.  I've seen what looks like small white flakes come out of the vent.  I know there has been a change in diet because the acquarium does not the algae that was in the pond and the supply of insects is no longer available.  I feed it only goldfish flakes.  I'm really concerned!  The folks at the pet store could only tell me to put salt in the tank, which I've done.  Can anyone help me figure out what's wrong?!

 I totally agree with Nossie.  I feel your fish has a intestanal disruption.  He needs a better diet than just flakes like Nossie says.
Frozen peas, bits of orange, tubifex worms, blood worms, even bottom feeder algae pellets would be good.
He may be constipated so feeding peas for a few days with frozen herbivoire food from your pet store may help.
It wouldn't hurt to treat him with a parasite remover as well.
You also might want to add a sinking pellet for goldfish to his diet, a good quality one.  Alternate feeding all these suggested things and feed him something different every day.
Good luck!

Thanx Nicole!  I LOVE Aqueon fish food.  I feed it to both my Goldfish and my tropical fish too.  The sinking bottom feeder food is great too!  My fish go nutz for it.  I throw one in the tank about once a week.  It's a great nutritous treat for them!