i doubt it is him activly hunting. as clever as they are they are not that clever. i suspect what you are observing is territorial protection of his cave followed by oppertunistic feeding. rainbows and kribs are known for liking caves and if he spots on go into his cave he will get protective and try and take it back resulting in a fight which the other fish will always loose...even oscars know not to take on a bristlnose!
once the fish is dead, he prob thinks 'why not?...om nom nom nom'
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