The 63 liter tank is appx. 16.5G fish is appx 7 inches - Does this length include the tail?
There are quite a number of things to discuss.
The top issues are water changes and the size of the fish for the size of the tank.
So, right now, when was the last water change water in the tank?
Popeye is often caused by water quality issues.
I will add a bunch of links for articles to read through below. There will also be links for other articles as well similar topics in the articles themselves. The more information the better.
So if the water has been changed within the past week or 10 days, go ahead and take 50% water change of water only. And replace as you have before and always should with same tank water temp treated to dechlorinate.
Then do another 50% water change, water only, not the whole tank. The reason is that the difference in water in the tank and the new cleaner water from the tap is considerably different.
The double water change will make the change in the water more gradual.
If the last water change has been longer than about 10 days, do the double pwc
but 25% then wait a few hours do another pwc
, about 40%.
That will help reduce stress for the fish.
Goldfish are big waste producers and need a large tank, extra filtration and frequent water changes!!!
Realizing you already have a tank and fish, but this article has lots of important information, especially about the nitrification cycle. Which is about beneficial bacteria / BB, which builds a colony in the filter pad/ media making your tank able to process waste and excess foods and keep the tank more stable.
I will add more links for the articles in the next post.