Gold Fish Sinks

Question
Ah, I know that gold fish isn't listed as your area of expertise, but aside from another expert who specifically says not to ask about them, you are the only expert available and I am desperate to help my fish.

I won a fish at a carnival about a year and a half ago, a very  tiny feeder goldfish, about the size of my thumb, and have been keeping him in a 15 gallon tank. At the same time this year, about half a year ago now, I won another fish from the same place, again a tiny feeder goldfish. This new gokdfish was kept in a separate tank for a few days and then introduced to my old fish. They both got on fine and had been doing well.

The first fish had always acted a bit odd, sitting at the bottom of the tank like he was dead until some one came near and then swimming away, or just bobbing up and down almost completely vertical, either nose up or nose down until, again, some one would come near and he would swim around like a normal fish. But in the last week he only sits at the bottom, sometimes tilted on his side and his fins are flat against him, his tail is all drawn up into a point, and his once larger mouth seems to be sucked in. He doesn't even surface for food or chase around the tank. Unlike my newer fish, he hasn't grown at all in the 17 months I've had him, compared to the newer fish almost doubling it's size by now. He also seems top heavy and isn't moving barely at all.

The new fish still seems to be doing fine, but will sometimes rest right next to my sick fish when I turn out the aquarium light for the night.

I tried asking the veterinarians office near me but they replied goldfish can't live more than a year. I know this is untrue because the very first carnival fish I won, also a tiny feeder gold fish who grew to be the size of my hand, lived more than eleven years, in the same tank I have my two current fish in.

The tank has no heater, but a fully working filter and I always use stress coat when I clean the tank.

Please help, I don't know what to do to help my fish, I've never had a sick one before.

Answer
Hi Kaitlin,
Goldfish aren't my expertise, but I will try to help you.  It isn't true that goldfish only live one year, they can live to a ripe old age.  The fish will not grow bigger because of the size of your tank.  He will stay that size, unless you put him in a bigger tank.    Check your water to see if you have any ammonia in it.  If you do, make water changes everyday until you get rid of it.  Change 50% of your water everyday.  When your water chemistry is okay, buy Malafix, and follow the directions on the bottle.  Make sure you shake the bottle well before adding the medication.  Never heat the water with goldfish, they are cool water fish.  3 teaspoon of aquarium salt may be added to your aquarium.   I hope this helps you.
Lynda