Usumacinta

The firemouth cichlid, Thorichthys meeki, is a Central American cichlid from Mexico's Atlantic slope and the Yucatán south into Belize and Guatemala. A popular aquarium fish for a long time, it is almost always commercially raised. It is easily identified by the bright red or orange underside of the head, which is more pronounced in adults. It is not especially aggressive but becomes territorial when breeding.

The green swordtail, Xiphophorus hellerii, is a livebearer native to southeastern Mexico, Guatemala, Belize and Honduras. Among the world's most recognisable aquarium fishes, it is ubiquitous in the trade, where selective breeding and hybridisation with X. maculatus and X. variatus have produced a bewildering array of ornamental forms. The name is often misspelled X. helleri.

The Mayan tetra, Hyphessobrycon compressus, is a characid from southern Mexico, Guatemala and Belize. The type species of the genus Hyphessobrycon, it is not well known in the ornamental trade. Analysis has shown it to be more closely related to 'rosy tetra' species such as H. eques and H. pulchripinnis than to many other members of the genus. It has been collected from rivers, streams, temporal swamps and permanent lakes.