This happened 5 years ago.gotta disagree Jessica....have seen all of mine, over 30 yrs munch on earthworm pellets...maybe it is in the filler as in other foods that make them go for it. Veggie for sure as a mainstay, but feel they can be Omni as most other fish are.
Mine LOVE freezedried blackworms!I've certainly seen Ancistrus go after various pellets, flakes, even frozen bloodworms and such...I wouldn't say that just because your particular BN was comically uninterested that they are then strict vegetarians. And keep in mind that even algae wafers (probably like the one yours devoured) have fillers that are NOT from plant matter.
But as Rb said veggies are the best first option.