Rosys are as dangerous. But the easiest grow your own in our climate (we aren't in Australia) doesn't work. Tilapia have millions of quick growing food fish, but they also have spiky defences. Livebearers would take months to get to food size, and a ton of space.
It's a catch 22. Feeders are dirt cheap, and to be dirt cheap, they are badly treated.
The only obligatory fish eater I ever kept were pike gouramis, Luciocephalus. I couldn't coax them into eating anything that didn't swim. I had to buy guppies, and the cause of death for my gouramis was a bacterial disease caught from feeder gups.
I wonder if there's something in the colour? Rosy minnows are a colour sport, a mutation of an olive green fish. Goldfish are the same. Does the trade use them because their colours attract predators? They are in the same colour range. I don't like feeding fish to fish, and don't generally go in for fish that need other fish to eat (I was doing a book about gouramis when I got the "pikes", and wanted hands on experience before I wrote). But you raise a question there.
I used to live beside an army base, and the soldiers bought piranhas like they were going out of style. There was one local store that had piranhas, but also had tank after tank of red swordtails. Only old fashioned brick red swords. Again, the colour question...
on a diet of predominantly gold fish
