goldfish tail and activity

Question
I have a goldfish that I won at a local fair. When I first brought it home, it spent all of its time sitting at the bottom. I looked it up and it apparently had swim bladdar disease, so I followed some tips on the internet and it appeared to work. Then, a few weeks ago, I noticed its tail was gone and that in its place was a black stump. Now the stump has turned white and my fish spends most of the time either sitting at the bottom, floating like it's standing on its head, or moving the back end of its body furiously in an attempt to move. I clean its tank and replace its water every week, I keep the tank out of direct sunlight, and I feed it once a day. I know it's not normal, so what's wrong?

Answer
his tail probably burnt of with all the ammonia in your tank, goldfish produce alot of waste and simply cannot survive in bowls, especially not wothout a strong filter, i know many people probably told you they could, but you have living proof there wrong now dont you?

also dont replace its water, do a 25% water change weekly

also you could get some fin rot treatment to see if it helps, but dont put too much in, it sounds like his tail could have burnt off from all the chemicals you have used if it isnt ammonia.