I'm going through the same thing!
My tank came with four 3" or so gold barbs that have lived with my oscar since he was maybe 3 1/2", hes pushing 6 1/2" and the other morning i woke up to see my oscar not as crazy as normal, a missing barb, and an oscar with a full tummy. I thought okay nbd that was the smallest or second smallest one, the rest are too big.
2 days ago I woke up to see 2 barbs, a second look and its
2 1/2 barbs.
I saw a gold barb with its head and half its body bitten off laying on the gravel with its fat and tissue hanging out like cotton balls and its intestines and guts spilling out when I netted it out.
I found that my oscar would never pull something like that when I'm watching him or the lights on. I hid on the staircase, turned off the lights, and saw him start hunting.
For the last two nights I've left the lights on overnight and no casualties
Funny thing is, he never even considers going for the opaline gouramis who aren't even 3 inches. I've found he never EVER messes with a fish that can't eat pellets.
He chases the GT and leptobarbus hoevenii around the tank all the time because they can actually compete for food, he occasionally messes with the barbs because they can grab pellets sometimes if they try, but he'll never mess with the gouramis because he just watches them sadly trying as hard as they can to pick at cichlid bio-gold pellets triple the size of their mouths.
When a fish isn't worth a meal, or isn't competition, lots of fish won't bother.
Rant over.
