I can rule that out at least. The head shape, whiskers, and dorsal fin are all wrong. Plus piraiba can grow 8-10', and most pimelodus grow to 12"-16" max.
It looks just like my big guy before he lost his pattern and became mostly silver. Also, keep in mind that there are a lot of regional varients. I have a shoal of Pimelodus blochii. Its awesome the way they school together. Here is a picture I took yesterday morning. Sorry about the flash, it was dark and I didn't want to turn on the lights.