i doubt mata matas could eat any fish so far said. definitely not discus, if big enough. as discus are too wide to fit in there mouths. mata matas don't have teeth! they are perfectly harmless unless to anything that is maybe as small as a guppy. not even baby mata matas may be able to fit a large guppy in their mouth!
and i 100% ensure my mata mata can ever eat my stingray that is his size. he has not teeth!
he's extremely peaceful with all my fish, never shows aggression and doesn't mind my fly river turtles digging under him or sleeping on him.
and my mata mata is pretty big, yet the most peaceful fish in the tank.
never overestimate turtles just because they look scary