My one and only goldfish has always lived in a 100L tank. I have never had any problem with his health before all this happened. I have started on a programme of 12 days antibiotics, which I am hoping will help Charlie my fish. I have done regular water changes and always checked the temperature of the water I have replaced in his tank in the past, so I do not know why this has happened, but I must have done somthing wrong or he would not be sick. I am just keeping my fingers crossed.