If these fish are considered a pred than a pacu is a pred right? LOL they are omnivorous they might snatch the occasional fish but eat a primary diet of vegetation. You will not see a true predator eating cucumber or algae ----sorry---------fei fengs are just opportunistic feeders at best.
fei fengs will clean your tank of algae and eat algae pellets and other veg. matter. but like pacu's as stated above they will eat it if it can fit in there mouth. also i have read that they show quite a bit of con-specific aggression. so only get one.