Goldfish don't make great food for most predatory fish anyway, they don't make very nutritional food and are rather slimey and fatty fish, which some predatory fish (particularly tropical predatory fish) can find difficult to digest.
If you buy any store bought feeder fish, you should always quarentine it for at least 2 weeks to help avoid stuff like this.
More than often though its better to breed your own feeders, this will also help cut down on the chances of the feeder fish passing stuff onto your prize preds. Convict cichlids make very good feeder fish, easy to breed and they also breed on a large scale (lay hundreds of eggs which hatch in 3-4days), grow quickly too- all you need is a 20-30gal filtered and heated tank with a pair of breeding convict cichlids and a fry tank, and you have a pretty much never ending supply of fry...