+1 for commercially bred feeders not being the best option. When they get shipped to stores, they sometimes have hitch hikers like tadpoles, crays, beetles, larva, and plants (once we got a couple tilapia fingerlings.) Who knows what else they can come with that we can't see.
It sucks, but you could buy a 75g and stock it with a sustainable population of guppies. Along with being safer, they'll be more nutritious than feeders if you feed them good food.
Have you tried frozen minnows?