I have kept "sabre tooth barracudas" H. scrombroides (aka: 'payara') as well as Rhapioiodon vulpinnus before in the past. They are extremely delicate sensative fish. Be sure your water conditions is in tip top shape at all times, high oxygen/aeration in your tank, and a strong fast current flow (using powerheads)... THIS IS AN ABSOLUTE MUST!!
These fish stress very easy... so I suggest you keep them alone w/ no tankmates at first. They also have extremely fast metabolism, so you want to make sure they are eating good and well at first. Getting them acclimated and eating healthy is very important. Feed first with live foods... (feeder fish, earthworms, etc.). After they become stable eaters, try offering other foods (non live) as well. GL!
I had mine taking various foods... live rosy reds, raw shrimp, frozen (thawed) fish, earthworms, freeze dried krill, and even lrg. floating pellets (soaked in vita-chem).
see pics below w/ tankmates and feeding on freeze dried krill.