I tried putting some guppies with it. But the guppies ended up biting its long barbels.Disclaimer: Not sure if this is recommended at all due to the risk of disease from live fish, etc...But,
When I was traveling in the Peruvian Amazon, many fishermen keep several Brachyplatystoma species in tanks when they are quite small 1.5-3 inches in order to grow them up to optimal shipping sizes. To feed the fish, they simply keep a handful of small livebearers (1/4-3/4 inch) in each tank...they are locally abundant and apparently closely mimic the diet of juvenile brachies in the wild.