I came across an NPR story today while searching for goldfish information. It explains a surgical procedure done on a 4-year-old goldfish who had buoyancy issues. He was a chronic "sinker", meaning he had a hard time lifting himself off the bottom of the aquarium. I have had a fish like this in the past, and her issue eventually got so bad that her quality of life had deteriorated considerably, and I had to euthanize her.
Many people have tried various different ways to make slings for sinker goldfish to help keep them more buoyant in the water, but all types of slings eventually cause irritation and chaffing and thus are not good for long-term use. In this case though, rather than using a sling, a plastic tag was inserted under the fish's skin and a cork was attached to this tag. This held the goldfish up off the floor of the aquarium so it could once again swim normally (or close to normally at least).
Apparently a while after the procedure, the plastic
tag they used came out of the fish. So they performed the procedure
again, but this time using a tag that biologists use to tag wild fish
with. I have not heard anything about whether the second try was
successful long-term or not. If it was successful long-term, I think this may possibly be a good alternative to slings which can be irritating.
Here's a link to the story, to listen click the little icon next to the word "listen".
NPR "Fish Medicine"
Here is the goldfish during the procedure. The tag is being inserted on the dorsum of the fish; next to the dorsal fin.
This image shows the fish after the procedure. You can see the cork floating above him, helping him remain upright in the water and stay off the bottom.