One of my fantail goldfish doesn’t swim and just lies on the bottom, Is there help for him?

I have a 50 gallon tank, two black moors and two fantails and one small algae eater. The one fish, Wyatt, has pretty much had swim bladder disease since 2007. It wasn’t until I moved them to a 50 gallon tank that I noticed it more often. He has learned to live upside down most of the time and other times he was fine. Recently he doesn’t float anymore, he is on the bottom breathing heavily and occasionally swims erratically up for air or food. I put a tomato basket in the tank and he knows to go there to be alone. In the last week, his beautiful white fantail is shredded and and tattered. He still eats but I don’t know what to do. I thought of how I could attach him to a half cork so he would at least be upright and I could hand feed him. And I’ve come close to flushing him to save his misery. Every morning he is still breathing and seems to be a fighter.
I recently cleaned the gravel and did a major water change after medicating it for worms and then treated the new water with blue ick drops. It looks as if he has fin rot from the stress. The other fish are doing well and they even hover above him sometimes as if they are looking after him. I’m lost as to what to do.
He was like this before the medicating and water change, that’s why I did the water change.
I used Jungle Parasite free and then after that cycle was finished, I cleaned the tank and did the water change because there were nematodes in the tank.

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