You can feed them meal worms, i've have fed mine a variety mixed diet. Red worms can be soaked in water to get it's protective mucus off. Then again worms as in night crawlers or red worms if they are not interested in them asap they burrow into your substract. Keep an eye on what they like & don't like for future preferences.
I have tried wax worms they just suck them in then returned out the mouth. But def try super worms & meal worms. As for diet never nothing steady beside white meat/fish ex. like smelt talapia fillets ect., but would advice if fed these too cut them about the size of the piranhas mouth.
Only because they will shred them while eating especially in shoals, then you'll have plenty of cleaning to do. Or live feeders mainly minnows @ LBS way fresh than a LFS/rosy reds minnows as a treat or something more natural/simulate as they would be in the wild.
