The eating process, as seen in the videos below, is much less of a chomp-down than it is a suck-in. As the fish quickly open their mouths, it creates an area of low pressure (in the mouth cavity) to which the surrounding water rushes towards. As long as the smaller fish are within this zone of rushing water, they are basically sucked into the mouth like a vacuum cleaner.
A vacuum, coincidentally, works the exact same way, just with a different fluid: air. The spinning mechanisms within the vacuum create areas of low pressure to which the surrounding air rushes towards. Any dirt of debris in this zone of rushing air is then caught up.