Probably a Leporinus species, from what Gery calls the "Leporinus friderici group." It certainly does bear a resemblance to certain cyprinids, but none of them have an adipose fin so that brings us to the characims; catfish with adipose fins are usually fairly obvious as catfish, and I doubt this would be a salmonid. The inferior (or sub-terminal) mouth is distinctive, suggesting a substrate feeder or grazer.
There is a photo in Gery's book of the characids that is strikingly similar to the fish pictured in post #1, and Gery labels it as an unidentified species in the L. friderici group.
Gery writes (1977) that the Leporinus genus is one of the largest genera among the characiformes but one of the least known. He mentions some 50-60 probable species, and today Seriously Fish and Fishbase mention 90 valid species which are beginning to be divided up into other genera [ref. Sidlauskas and Vari, 2008].
Byron.