Goldfish with bulge near tail

[CDGQY]

Hello. I’ve had my goldfish for a few years. They live outdoors in a 100 gallon water trough. Late August I noticed one of my fish had a discolored bulge near his tail. I left him in the tank a few days when it was first thought to be a contusion but then moved him in to a 20 gallon hospital tank with an Aqua Clear HOB filter where he’s been (alone) ever since. In the ensuing months, I’ve tried treatments including erythromycin and general cure, always attempting to do frequent water changes. Lately, I shoot for 2 to 3 water changes a week and dose with melafix between changes. His appetite is good and he is active, but I think he is uncomfortable since he usually has his tail over an air stone that creates an abundance of tiny bubbles. Does anyone have a possible diagnosis or thoughts on effective treatment?
The last photo is from early September just before I removed him from the community tank. The other three photos were taken today.

 

[BBradbury]

Hello CD...

Unless you have experience in fish biology, you can't be sure if the fish has a legitimate problem or not. Putting chemicals into the tank water is never the answer. I would dose a little standard aquarium salt. A couple of teaspoons in every five gallons of treated tap water. Water changes are good, but one every few days is enough to keep the water livable for the fish. I would start feeding a little Spirulina once a day. It's very nutritious and the combination of salt and added nutrients should restore the health of your fish. Who knows, this could be nothing.

B

 

[CDGQY]

Thank you.

 

[libertybelle]

Could be a tumor. Goldfish can get them too.

If it’s that not much you can do without a vet involved. Sorry that’s probably not particularly helpful but that’s what it looks like to me in your photos.

 

[CDGQY]

Thank you, LibertyBelle. I was also concerned that it might be some sort of tumor. However, the skin(?) surface seems to be affected as well, and I had assumed a tumor would be subcutaneous. If indeed it is a tumor and nothing infectious, then I would like to return him to the big tank so he can live out his life with his mates. It might be a good way to begin the new year.

 

[libertybelle]

Thank you, LibertyBelle. I was also concerned that it might be some sort of tumor. However, the skin(?) surface seems to be affected as well, and I had assumed a tumor would be subcutaneous. If indeed it is a tumor and nothing infectious, then I would like to return him to the big tank so he can live out his life with his mates. It might be a good way to begin the new year.

I’m not sure there’s any way to know for sure. The safeish course might be to reintroduce him to one fish to test for contagiousness. If after a few weeks or more the new fish is fine it’s likely not a contagious malady.