Cyprinodon pachycephalus

Cyprinodon: from the Ancient Greek κυπρῖνος (kuprinos), meaning 'carp', and ὀδούς (odous), meaning 'tooth'.

pachycephalus: from the Ancient Greek παχύς (pakhus), meaning 'thick', and κεφαλή (kephale), meaning 'head', in allusion to this species' pronounced head shape.

Type locality is 'Baños de San Diego, east of Chihuahua, Mexico, 28°35.5'N, 105°32.5'W, elevation about 1140 meters', and this species is endemic to the spring of San Diego de Alcalá plus its distributaries in Chihuahua state, northern Mexico.

The springs at Baños de San Diego are thought to have originally flowed into the nearby Río Chuviscar but currently terminate at a man-made impoundment with the natural stream formed by the spring outlet having been diverted into a canal sometime during the 1970s.

The springs rise from circular sources around 20 cm8 in in diameter and flow to form an unshaded stream measuring 2-3 m in width and just a few cm deep.

They are thermal and produce water at a constant temperature of 43.8 °C111 °F although this cools to around 38 °C100 °F approximately 20 metres downstream and a temperature of 25 °C77 °F has been recorded in the impoundment pool, although presumably this fluctuates with air temperature.

C. pachycephalus is found throughout the system and in the lower part forms natural hybrids with the congener C. eximius, with other sympatric fish species in the impoundment including Gambusia zarskei plus introduced Lepomis megalotis and Tilapia.

It's unclear whether C. eximius and C. pachycephalus originally occurred together or artificial modifications to the springs permitted invasion by the former.

An aquarium with base dimensions of 60 ∗ 30 cm24 ∗ 12 in or equivalent is just about acceptable for a single pair but like most members of the family Cyprinodontidae this species does best when maintained as a group in a larger tank or container.

Need not be too complicated so long as there are plenty of broken lines-of-sight.

Provide plenty of cover in the form of aquatic plants, wool mops, etc., and if using filtration air-powered, sponge-type units are best as these will not harm eggs or fry.

Lighting is unimportant but can be used if you wish, and growth of filamentous algae should be encouraged if possible.