Couple of problems. The small size of the tank is one--one goldfish would be cramped in there, much less one goldfish and some additional fish. Second, if the tank wasn't cleaned for several months, and then a large water change was done, the sudden change in conditions can harm the fish. If the cleaning was too 'good', ie, complete tear down and refill, the benficial bacteria are damaged, and the sudden parameter change is further complicated by ammonia and nitrite spikes. Small tanks must be maintained regularly and frequently to avoid sudden changes.
I'm sorry, but really the best option will be to get a larger tank, and take care of it properly. Goldfish are prone to tumors, and keeping them in cramped, dirty tanks will not help. There isn't much that canbe done for tumors--surgery is an option, but only for the experienced keeper, and not something to be done casually. Improving tank conditions, and keeping them stable, will do worlds of good.