We have two goldfish.
At one time we had six but now only two are left. Some got sick and some got old (maybe).
We noticed that one of our two goldfish had started floating. It tried to dive, but it seemed difficult and it kept floating most of the time. We stopped feeding them for a week and it became OK, so we decided to give them less food.
After some time, the same fish, instead of floating, sank to the bottom. It looked sick. When we fed them, it tried to come up to eat, but soon went back to the bottom.
We checked the internet and found that some salt in the water might help. We added some salt, that we had bought earlier, to the water.
The next day it started swimming and after a few days, it seemed to be recovered.
We hope they will live a long time.