Feeder goldfish tend to be normal common or comet goldfish, nothing different about them really, however they grow very large (12-15inches long usually) and so need a decent size tank of at least 75-100gallons+ or a good size pond, if properly cared for they will live between 15-30 years on average (there was even a world record winning goldfish, which was just a standard fair ground common or comet goldfish, which lived to the ripe old age of 47 years old).
Common livebearers like guppys and certain cichlids are often sold as feeder fish because they are prolific breeders and easy/cheap to produce, but they grow smaller than goldfish and so are easier to accomodate.
Although fish may be cheap to buy, my belief is that if you're going to use your fish as a feeder fish, get it over and done with quick in a humane manner, but if you want to keep the fish as a pet, you should supply it with decent size accomodation and a clean tank and good diet etc like you would with any other fish. I personally see all fish as equal, and that a feeder fish deserves the same amount of care and respect as any other fish etc.
The only problem with a lot of feeder fish sold at petshops is that they tend to be riddled with deseases and parasites since they are so cheap and so staff don't put that much effort into their care, obviously this is a problem though as you don't want to pass such deseases/parasites onto your prize predatory fish by feeding them dodgey feeder fish. So a lot of people either quarentine such shop bought feeder fish for long periods of time before feeding them to their fish, or simply breed their own feeder fish (which can work out to be a lot cheaper in the long run).
But anyway, thats all in the past, I'm very happy with my GSP.
