for one, please invest in a big tank as soon as you can. if the Goldie is a fancy, they need 30 gallons at the least, and a common/comet goldie gets to be 18-20" and need a much bigger tank.
goldfish are poop machines, and need at least double the filtration like a good canister or a good bio-wheel. (i.e. if the tank is 30G, you need atleast 60G filter.)
my 6 goldfish are young still, and are in a 65G w/ 140G worth of filtration. and they are getting close to needing at 120G soon.
invest in a liquid test kit, like API's Liquid Master Test kit.
my one comet is 6-7" and is about a year old tops.
the fish should not have clamped fins, they should swim around with all their fins out, not tucked in towards the body.
one, its stressed for sure... ammonia is most likely slowly making it sick and killing it.
please just look at my avatar.
also, FYI, a comet/common goldfish can out grow even a 30G very quickly. keep that in mind.
if you dont have room for a big tank, please do the right thing for the fish and give him to someone who has a goldfish/koi pond, or has a big Goldfish tank already, or give him/her back to the LFS.