Gold fish injury

Question
Hello, i have a 6 gallon tank, nothing too big.  It has been a established tank for about 8 months now.  I do not test the water often anylonger since the fish are always in healthy condition.  I change the water as needed to be done and clean the filter as well.

Ok so here is my problem....

I have a puffy cheecked gold fish that i love!  He was doing great aside from the occasional nibbles on his tail by the Beta and another fish (i dont know the name of it) It is long thin and silver.  A Bit mean at times but mostly chases the fish and wont hurt them.  

So, the gold fish at some point last night or earlier today was hovering and barely moving at the top of the tank.  I took him out into an isolated tank.  He has a crooked back now and is swimming upside down.  It looks like one side of him has bulged out toward the tail on his underbelly.  I dont think he is going to make it.   I was wondering if this is an injury or some internal disease.

I need to know because i have to deside if i should take those 2 aggresive fish out of the tank  for good.  I really liked that fish and it is ashame to see him die like this.  He is still alive but swimming upside down and not using all his fins.

The fish arent any bigger than he is, actually they are some what smaller only 75% of his size.  I am not entirely sure that they may have been able to do that to him.

Let me know,

Sicnerely, with high hopes

James T
JTJIVE@yahoo.com

Answer
Hi James,
Your goldfish probably wont make it. If you want to diagnos your fish with a disease I'll give you a website http://www.tropicalfishcentre.co.uk/Diseases1.htm
I dont have enough details to diagnos it myself but your fish sounds big so he could just be dieing of old age.
         I hope I helped you,
                  Samantha