well is almost like a gamble question there Innes. There is not any exact diet that you could get all P researchers to agree on... so I will give you a list of things that they eat in their wild environment: they will eat nuts, fruits that fall from trees as pacus do in their diets and they will also chomp on scales and fins nipped from similar fish as tetras at a younger age...
...but for aquarium conditions here are some foods that are tested to be suitable:
frozen foods such as brine shrimp, whole bait minows because they chance less disease, frozen krills and even Euphasia supperba and Pacifica, lean meat as beefheart that has been trimmed properly, flakes and other commercially produced diets are reccommended as well, freeze dried foods such as blood worms work well too...
...this is just a starting point.... the chances are that you are already feeding you P sufficiently anyhow
