He doesn't have bouyancy problem. So I think it's the bubble that sits in his stomach. Now he developed swollen on half of his side. Look like dropsy but just half of his side bulge out.Sorry, not exactly sure what happened, but I hope that your fish improves soon.
Treating Hexamita aka Spironucleus with Epsom Salt Solution
Use the original batch, no need to refrigerate. Should remain stable for a long time, it's just a mixture of water and magnesium sulfate.