Baryancistrus niveatus (Castelnau 1855) (tentative ID by NealeM)
To about nine inches in length.
pH range: 6.5 – 7.0; dH range: 8 - 10
South America: Tocantins, Xingu, Tapajós and Trombetas River basins.
http://www.fishbase.org/Summary/SpeciesSummary
.php?id=50006