I figure most of us won't touch the more acidic ants anyway, so the ones we can handle make good food. Analysis of the stomach contents of wild-caught killies and small tetras/barbs show ants to be a staple food of many small fishes. Bettas probably eat their share - they always loved eating black ants here.
I've fed them swatted mosquitoes (their favourite), wingless fruit flies, small spiders, moths and every other small thing I have to swat or crush because my kids insist. I've also seen them hunt small bugs that landed on the surface of their tanks.