It is usually not even close to worth it to breed goldfish, guppies, etc. for feeders. Not for any moral reasons, simply the reasons of growth rate
and the scale of operations
you need to make it worthwhile at all.
I've tried this a bunch of times and it's a great idea to have healthy and disease free feeders. The problem is, unless you are keeping just one small predator and have lots of tankspace for your feeders, it's not worth it. Most people prefer to use 80% or more of their tankspace for the predatory fish they keep and less than 20% for feeders. It has to be reversed
if you want any kind of continuous supply of live food.
For those interested, I got a good amount of guppy fry by switching my stock of guppies from a 40 gallon to a 100 gallon. Lots of fry were dropped and, naturally, they had a ton of space to grow in with minimal nitrogenous waste. I even removed the adult guppies to enhance this. The end result is about 30 guppies that are still less than a centimeter in length over a month later. I'm guessing everyone can extrapolate from this how futile the idea is, even at a 100 gallon scale!
My best bet has been to catch or buy feeders (mosquitofish, rosy reds, minnows, etc.variety is best) and to feed them healthy foods during a quarantine period (to prevent disease transmission). You'll be much happier using that approach. Hope this helps!