Looks like dropsy which is rarely curable, most don’t make it last a few days. You can try to dose Epsom salts at rate of 1 tablespoon per gallon slowing increasing over 8 hours and dose methalyne blue(stains silicone). I’d personally take a mallet and put the fish down. Dropsy is not a disease but a symptom of a larger problem such as a bacterial infection often caused from poor water quality.