Goldfish and rosy red minnows can be easy to breed, but they suffer from being high in the chemical thiaminase. They must not be used as the staple diet, because over time they will prevent the predatory fish from having enough vitamin B1, which will cause serious health problems. The muscles, bones, and nerves appear to be most seriously affected.
Using cheap fish from your local pet shop is a poor alternative; by any objective standard the risks are high. Quarantining and treating feeder fish with anti-parasite and anti-bacteria drugs may be an option, but if you aren't prepared to do this, you would be wise to pass over this source of feeder fish.