Goldfish with swollen side

[Ojos0307]

Hi. My goldfish is swollen on its side. When it swims can't maintain balance and even goes upside down. It's eating well but I'm giving less food. What can this be and how can I treat it? Thank you for your help.

 

[Aiken Drum]

Please take a read through the "unhealthy fish" sticky and give as much detail as you are able.

https://www.aquariumadvice.com/forums/f17/before-posting-about-unhealthy-fish-read-this-32451.html

Can you see if the scales are "pineconing"?