I've been doing it on fish that either my sims caught when I need to keep them busy, or my cats have caught while I've been trying to get Fairy Damsels, Toads, and Luminous Salamanders.
That's pretty hilarious, a few rows of fish laid out on the floor, ready for conversion. I've gotten gems, metals, seeds, the fish I'm after. And if you don't like what you got the first time, reconvert it!