Rumblesushi, i think you're right. It does look like Cynadon gibbus sp.??
I've kept hydros, cynadons, and Rhaphioidon species before in the past! Very cool entertaining fish during feeding times, but are fairly sensative skittish fish. Don't house them with any "mean or territorial" fish that can stress them, and also be sure to keep a good strong current flow in the tank. try to feed as much as you can possible... these fish have extremely fast metabolism and will eat as much as they possibly can fit in their stomach. Mainly feed on live foods, but it is possible to get them on non-live as well. I had mine taking freeze-dried krill, cut raw shrimp and even floating pellets,,, along with the regular diet of live feeder fish and earthworms/nightcrawlers.. (see last 2 pics below... vulpinus taking fd krill).
I plan to keep a group of payaras & vulpinus again sometime in the near future... i'm curious to see what size i can grow them out to?... but who knows?? Anyways, GL and enjoy!
actually i dont have much experience with them but i do know that hydrocynus armatus is anther form of a vampire tetra that doesnt have the weird hunting style where it swims verticle and the armatus gets about 2 feet