Feeder goldfish is one of the worst live food you can feed to your fish. They come with diseases as they are very weak (mutated and deformed from their original genes through centuries of breeding by humans). Their slow swimming and water resistant non-streamlined body shape also make them easy targets for parasites to latch on to.
If feeder goldfish is to be used, then you should quarrantine them for as long as possible. The longer you quarrantine, the more chance there is that the sick ones can get more sick as to be distinguisable from the healthy ones and enable them to be removed. Obviously, you will need to quarantine them in a good environment so that they don't fall sick even though they are healthy! Keep the water in good condition by daily water changes if you do not have a ready cycled tank.