Goldfish missing back fin

Question
A while back I rescued 57 goldfish from my work Christmas party. Although I
did not have the ideal space for them, I could not sit back and watch them
die
(I had watched my boss eat one alive at the party and this is scarring
enough).
I hate to admit this, but I only had a 10 gallon tank for them. I have since,
however, transferred them to a 30 gallon. It has been over three months now
and I am sorry to say there are only 24 remaining. However to keep myself
from writhing with guilt, they would all be dead if not for my taking them
home.
My concern is that I noticed a couple of days ago that one fish is now missing
his back fin, and he has several torn and blackened scales. Since I discovered
this I have noticed he keeps to the bottom corner of the tank behind an
ornament, and when he comes out to eat the other fish chase him away. Do
fish exile?
In the past I have found several smaller fish dead and picked apart (although I
pray the eating happened after the death). Has he been picked on? Or could
this be a disease?
I took him out of the tank tonight and he is in a bowl. Should I find him an
environment of his own or can he go back in the tank?
(I know I may not seem like the best caregiver, i.e. the overcrowding, but I
love these fish with my whole heart and simply don't have the money to
provide them with proper space. But I am trying to convince my parents to
build a pond in the spring!)

Answer
Hi Natelie:
Your fish has black spots across its body because it is recovering from ammonia surges in the water. This is when the fish is burned from the high level of ammonia and/or low PH. It is very brave of you to take on such an adventure of this many fish. Your fish with the absent black fin probably has scale and fin rot. This is a bacterial disease, and can be healed by changing 25% of your water every week. This will provide better water conditions for your high capacity of goldfish. Test your PH and ammonia level. What kind of food are you feeding your fish? Check if the fish looks like it has been sprinkled with salt, which could be fish parasites. If any fish look like they have parasites or have abnormal behavior, please remove them from the tank and let them swim around in a "quarantine tank", maybe in the tank that you abandoned-the 10 gallon. Keep the fish in a separate tank. The reason that the fish may be hiding would most likely be a swim bladder infection. The cause of these would be improper nutrition or water. Again, test the PH and ammonia levels to see about the water. Try feeding the goldfish thawed out frozen peas.
The other fish may have noticed that the hiding fish is weak and sick, so they may start to nip at it. In the case of the three dead fish, the little fish were infected by disease and then the other fish picked the carcasses clean AFTER DEATH. I wish you good luck, and if any more problems persist, ask me more questions!! Thanks---
Sincerely,
Jenny