From personal experience, in the disease categorey, this is 99% on the fish keeper. Sure your fish can get diseases no matter what you do to keep the feeders healthy... It happens. About all of the videos on YouTube of people feeding live feeder fish show them going straight from the plastic, oxygenated bags from the LFS, to the fish tank. IMO this is why there are so many doubts on feeding these goldfish, because people like that have their fish killed and wonder why, blaming it on the risky feeding of live fish when they are the ones that neglected to quarantine the feeders to reduce the risk.