No, bristlenose plecos will not eat fish. No matter how big they get and how small their tank mates are, bristlenose plecos will never hunt down and eat other fish.
Bristlenose plecos are predominantly herbivores. Approximately 75% of their diet consists of algae and plant matter. They prefer to eat algae wafers, spirulina tablets, chopped-up vegetables, and occasionally, frozen bloodworms and brine shrimps as well. Bristlenose plecos even love to nibble on wood to supplement their diet.
With that said, these bottom dwellers are opportunists, so if a fish dies and sinks to the bottom, they’ll gladly eat it.