I've never tried to treat for this...originally I didn't know what it was, but after a few autopsies finally figured it out. I haven't read of any treatment that fixed this, so I never tried. I have read of using salt because that does effect fluid movement. Seems to be a "salt affects fluids" "this is a fluid thing" so it should work. But I think it's a genetic thing kind of to be expected with how these fish were created. If so, then salt might change the symptom but I don't see how it could fix the problem. Just my thinking, not saying it's right.
I just let them live whatever life they get and put them down when they can no longer swim. Fish with a fluid problem always seemed happy and hungry right up to the end. They can live more than a year, or blow up pretty fast.
Another thing lots of people think that kind of swelling would be is constipation. People like to feed mashed peas for that. But to me, if that's from constipation I'm thinking the fish doesn't have long to live as it would have moved beyond simple constipation. But some fish like peas so that couldn't hurt.
The last thing people will often say is that this is dropsy. But dropsy isn't a disease. Dropsy is just another way to say "bloated fish".