The catfish appears to be suddenly trying to attack the piranha... they have always gotten along, nothing's changed, nobody's gotten any bigger (both are terribly stunted - I'm working on that)... I'm sure they're not fighting over the food itself because the catfish has always had a stealth eating style and just eats whatever is leftover off the bottom (I honestly never knew it was eating the food until very recently). They have eaten flake food their entire lives (I will be changing that soon). They were matched as tank mates by pros at the fish store, though the pros don't know too much about piranhas in my state (shhh, I inherited them)... I know there are a LOT of problems with their current setup, but I'm not sure where the sudden aggression is coming from... and it's a super fast catfish, so the piranha really can't do anything but try to get away. She is getting old (like 10), and the catfish is only about 5. Maybe that's why?