Help !! My Goldfish Is Sick But I Don't Know Whats Wrong ?!?

I have a goldfish that is about two years old, and about three inches long. I've removed it from the tank it was previously in, about a ten gallon tank with two other small fish and about three months ago I had a fish die in it. Now my fish just floats on its side at the top or the bottom of the tank and floats around, like my other fish did before it died. The fish also seems like it's tail is fraying apart and becoming shorter. I don't know what's wrong or what I'm doing wrong, PLEASE help me I'm very distressed just over my little fish ?!?! :? Please Help Me ASAP
I Hope You Guys Can Help Me And My Fish

 

What other fish were with it and how big are they?

What kind of goldfish?

What size tank is the goldfish in now?

What are your readings for ammonia, nitrIte, and nitrAte?

What filter?

I'm guessing it's related to water quality since a 10 gallon is too small for a single goldfish, let alone with some other fish. And after 2 years, it should be bigger than 3".

-Chelle