Pencilfish

About

Pencilfish (Nannostomus minimus) are among the smallest members of their genus, native to the blackwater streams of the Potaro and Mazaruni River basins in Guyana. At under an inch fully grown, these are genuinely tiny fish, and that size dictates almost everything about how you keep them. They do best in a species-only setup or alongside other very small, non-aggressive fish. Think diminutive characids or small catfish. They also work surprisingly well as dither fish for Apistogramma and similar dwarf cichlids, since they tend to cruise the middle and upper water column and pose no threat to fry. Like most pencilfish, they swap their daytime coloration for a cryptic pattern of vertical bars after dark, and interestingly this color shift happens even in blind individuals, suggesting it's a hardwired rhythm rather than a response to light. Males spar regularly during the day, which is entertaining rather than damaging, and the species is gregarious enough that larger groups of ten or more bring out noticeably bolder behavior. Breeding is achievable in a well-planted aquarium, though fry are tiny and require infusoria-sized foods to start. The species occasionally appears in the hobby under a misidentification as the related N. marilynae, so it pays to know what you're looking for before buying.