I had one comet and one fantail that were in a 10 gallon tank. Right up front, I know this is on the small side. The original fish was a rescue (he and another fish were abandoned at my old apartment complex) and we took them both in, and both were small fish. We figured we could upgrade the tank as soon as we found an affordable one used. His tank mate died not too long ago, and ever since he was very lethargic and seemed unhappy. We chose to get him a small fantail goldfish to keep him company, with plans to upgrade the tank. This was after months of him hardly moving except to eat. He and the new fish got along great and he became much more active. A few days ago I noticed they were both sitting on the bottom for most of the day, but thought they were just resting, as there were no other strange symptoms at the time, and I knew the original fish often slept during the day. Two days ago, I glanced over at the tank and the original goldfish was floating sideways in the tank, very very dead. 
He had extremely fringy and shredded looking fins, and his coloration had paled, but that was all. He had not been dead for long, so decomposition seems like it isn't the cause, but I had not noticed either symptoms that morning.
Now I have the fantail remaining and he has gotten worse and worse, to the point that I don't know what he even has. He looked fine the morning we found the other fish dead, but by that evening he looked like he should be dead himself. Surprisingly, he was still eating up until yesterday, but spent the rest of the time at the bottom of the tank. He has spent the last two days sitting with his fins clamped, and he has what looks like blisters and fuzzy patches all over his body. In addition, he has what looks like an ulcer on either side of his body, but greyish-black in color. The wounds look almost like gashes, but since they are not red I do not know what the cause could be. I watched both these fish very often, and there were no signs of aggression to account for this. I do realize that the new fish might have brought some disease into the tank and infected the original (and this was a risk we weighed against the original fish living the rest of his life alone), but I would like to treat and hopefully cure the other fish if possible. It may also be important that the remaining fish has been losing color in his tail fin. It looks mottled and red at the base, and the outer half of his tail is just white now. It is like the pigment is breaking apart and dissipating. His breathing is not fast, in fact, the only way I can tell he is still alive is that he turns his eye towards me when I come near to check on him. His gills and mouth are barely moving, but he is not going to the top of the tank for air.
I am not new to fish-keeping but this has me stumped. I have never seen such an aggressive illness, and to be honest I was almost certain he would not survive that first night, but now I am racing against the clock to try to find out what this is and fix it before it is too late. What we have done so far is put a fungus medication tablet in the tank, Jungle friends I think was the brand we wound up using. That was two days ago, but he has not shown any improvement, which has got me worrying that it may be bacterial. I have no antibacterial on hand, so I would have to choose one to buy if that is the case. We also did a water change and removed the carbon from the filter immediately before treating him.
So to summarize, what could this possibly be and how do I treat it? Is it alright to treat with antibacterial and anti-fungus at the same time to cover for both possibilities? I don't want to have to remove him from the tank and refill it because I am worried that the added stress may be too much for him to handle, but there is already anti-fungal medication in the water. Where should I go from here. Thank you all so much in advance for your help! Since the illness claimed our other fish in a matter of a day, and this little guy is still holding on, I want to do everything I can to give him the best chance at recovery.
He had extremely fringy and shredded looking fins, and his coloration had paled, but that was all. He had not been dead for long, so decomposition seems like it isn't the cause, but I had not noticed either symptoms that morning.Now I have the fantail remaining and he has gotten worse and worse, to the point that I don't know what he even has. He looked fine the morning we found the other fish dead, but by that evening he looked like he should be dead himself. Surprisingly, he was still eating up until yesterday, but spent the rest of the time at the bottom of the tank. He has spent the last two days sitting with his fins clamped, and he has what looks like blisters and fuzzy patches all over his body. In addition, he has what looks like an ulcer on either side of his body, but greyish-black in color. The wounds look almost like gashes, but since they are not red I do not know what the cause could be. I watched both these fish very often, and there were no signs of aggression to account for this. I do realize that the new fish might have brought some disease into the tank and infected the original (and this was a risk we weighed against the original fish living the rest of his life alone), but I would like to treat and hopefully cure the other fish if possible. It may also be important that the remaining fish has been losing color in his tail fin. It looks mottled and red at the base, and the outer half of his tail is just white now. It is like the pigment is breaking apart and dissipating. His breathing is not fast, in fact, the only way I can tell he is still alive is that he turns his eye towards me when I come near to check on him. His gills and mouth are barely moving, but he is not going to the top of the tank for air.
I am not new to fish-keeping but this has me stumped. I have never seen such an aggressive illness, and to be honest I was almost certain he would not survive that first night, but now I am racing against the clock to try to find out what this is and fix it before it is too late. What we have done so far is put a fungus medication tablet in the tank, Jungle friends I think was the brand we wound up using. That was two days ago, but he has not shown any improvement, which has got me worrying that it may be bacterial. I have no antibacterial on hand, so I would have to choose one to buy if that is the case. We also did a water change and removed the carbon from the filter immediately before treating him.
So to summarize, what could this possibly be and how do I treat it? Is it alright to treat with antibacterial and anti-fungus at the same time to cover for both possibilities? I don't want to have to remove him from the tank and refill it because I am worried that the added stress may be too much for him to handle, but there is already anti-fungal medication in the water. Where should I go from here. Thank you all so much in advance for your help! Since the illness claimed our other fish in a matter of a day, and this little guy is still holding on, I want to do everything I can to give him the best chance at recovery.