I have seen the same time & time again, even with large older Panaque nigrolineatus. They would almost fight to the death over the carnivorous food, ignoring all forms of plant matter, or herbivorous based pellets/wafers. This of course doesn't mean that the fish knows best, they clearly don't and with a feast or famine DNA code going back hundreds of years, most fish in captivity will eat themselves to death if given the opportunity. But it does tend to demonstrate what these fish prefer as they mature, if given the choice.