I have a 20 gallon (76 liters) tank with 6 goldfish - all small sized. I have had this tank for around 6 months. I do weekly 50% water changes and feed them twice a day - mostly pellets and flake food. Sometimes I give them peas.
Just around 3 weeks or so I have this one shubunkin who has started behaving different. It doesn't eat. And it just stays at one corner of the tank for the most of the time. I thought it must have had some infection so I did more water changes, but it still hasn't helped. And most importantly the other fish seem to be healthy and lively, apart from this one. It also seems to have lost some weight. I have attached an image of it.
I am unsure how I treat this fish. I am looking for some advice. It would be great if I could get any.