Hatchetfish

By opencage - http://opencage.net/pics/large_19302.asp, CC BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=38753845 A schooling fish, so should be kept in groups of 6+ of its own species. Females are rounder-bodied than males.

https://www.aqualog.de/wp-content/uploads/lexikon/s11512.jpg A schooling fish, so should be kept in groups of 6+ of its own species. Females are rounder-bodied than males.

By Tsunamicarlos - PD from en:Wikipedia, Public Domain, https://commons.wikimedia.org/w/index.php?curid=800135 By The original uploader was Dulcimerist at English Wikipedia - Transferred from en.wikipedia to Commons by Common Good using CommonsHelper., CC BY-SA 2.5...

By The original uploader was Dawson at English Wikipedia - Transferred from en.wikipedia to Commons., CC BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=1997527 A schooling fish, so should be kept in groups of 6+ of its own species. Females get larger and are more round-bodied than...

http://www.tropical-fish-keeping.com/wp-content/uploads/2015/08/Silver-Hatchetfish-Gasteropelecus-levis..jpg A schooling fish, so should be kept in groups of 6+ of its own species. Females are rounder-bodied than males.

By opencage - http://opencage.info/pics.e/large_19683.asp, CC BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=38753845 By Neale Monks at the English language Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1997497 A schooling fish, so should be kept in...

By Brian Gratwicke - originally posted to Flickr as Thoracocharax stellatus (Spotfin Hatchetfish), CC BY 2.0, A schooling fish, so should be kept in groups of 6+ of their own species. Males are slimmer than females.

By Guérin Nicolas (messages) - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4320015 A schooling fish, so should be kept in groups of 6+ of their own species. Males are slimmer than females.

By Ictiologia Universidad Católica de Oriente - Gasteropelecus maculatus, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=53170053 A schooling fish, so should be kept in groups of 6+ of their own species. Females are rounder-bodied than males.

By Clinton & Charles Robertson - Flickr: Characidae Triportheus cf angulatus, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=16049196 By Haplochromis - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=5839869 A schooling fish, so should be kept in groups of...