The Coral Red Pencilfish (Nannostomus mortenthaleri) is a species of freshwater pencilfish. They are characterized by their distinct narrow "pencil-like" bodies, and distinct black, red, and white horizontal stripes. Males are more colourful than females, and these pencilfish will not gain their full range of colours until they're fully-grown.
These popular and colourful pencilfish are hardy, making them a popular choice for beginners and advanced keepers alike. Despite these fish staying relatively small (full-grown specimens generally reach 1.5" in length), a 20-gallon aquarium is recommended to keep them. These fish should be kept in groups of 6+. Coral Red Pencilfish prefer water pH between 6.5-7.5, and do best in a water temperature between 75F-82F.
Coral Red Pencilfish are micro-predators in the wild, but captive-bred specimens will readily accept both flakes and pellets. Their diet should be supplemented with high-quality live or frozen foods such as brine shrimp, bloodworms, or daphnia.
While these fish are hardy and boisterous for their size, they should only be kept with other small, peaceful fish. They may be easily stressed by tankmates that are too boisterous or active. Male Coral Pencilfish may periodically spar with one another when kept in a group with other males; while this is normal and not generally harmful, we recommend keeping a group of Coral Red Pencilfish in at least a 20-gallon aquarium to prevent them from seriously injuring one another.
Photo of a Coral Red Pencilfish by Brian Gratwicke - originally posted to Flickr as Red Pencilfish, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=8229251