I had a male betta awhile back that would eat nothing but young fry & he did good , caught every one, never stopped hunting till they were gone.
So later on I tried dropping fry in with some of my other males,
It got to where I THOUGHT the fry had been eaten to only see it pop out one day ALOT bigger,
I move things around here all the time, I have sororities of just females and then I have tanks with young guppy fry, adult guppies and their fry and I pretty much always have between 1 and 3 or so depending on the female bettas or 1 single male betta (sometimes a pr) in with most of my tank. None of mine will look sideways at a fry.
I bought a big female type of Cichlid, was told she'd eat my cull fry for me, Well that was a bust, Put a dozen fry in and pulled a dozen fry out a months later, She just sits under them watching them swim around.
Thinking about getting an oscar, maybe I'll have better luck with him eating young fry and keeping the numbers under control.
My recent bunch of betta fry are almost 2 weeks old now & Still with Dad,