Stocking Fish Tips: Slowly acclimate fish to your current setup or preferrably a quarantine tank. When bringing home new fish, dump the bag contents (fish and water) into a clean (used only for fish) 5-gallon bucket and then add about 1 cup of aquarium water to the 5 gallon bucket every 10 minutes. Continue to add 1 cup of aquarium water to the 5-gallon bucket every 10 minutes. After an hour or so your fish should be ready to add to the aquarium.
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The Mexican tetra or Blind Cave Fish (Astyanax mexicanus) is a freshwater fish of the characin family (family Characidae) of order Characiformes.  The type species of its genus, it is native to the Nearctic ecozone, originating in the lower Rio Grande and the Neueces and Pecos Rivers in Texas as well as the central and eastern parts of Mexico.
This fish, especially the blind variant, is reasonably popular among aquarists.
A. mexicanus is a peaceful species that spends most of its time in the mid-level of the water above the rocky and sandy bottoms of pools and backwaters of creeks and rivers of its native environment. Coming from a subtropical climate, it prefers water with 6.0–7.8 pH, a hardness of up to 30 dGH, and a temperature range of 20 to 25 °C (68 to 77 °F). In the winter it migrates to warmer waters. Its natural diet consists of crustaceans, insects, and annelids, although in captivity it is omnivorous.
The Mexican tetra has been treated as a subspecies of A. fasciatus, the banded tetra, but this is not widely accepted.
A. mexicanus is famous for its blind cave form, which is known by such names as blind cave tetra, blind tetra, and blind cavefish. Some thirty distinct populations of Mexican tetras live in deep caves and have lost the power of sight and even their eyes. These fish can still, however, find their way around by means of their lateral lines, which are highly sensitive to fluctuating water pressure.
The eyed and eyeless forms of A. mexicanus, being members of the same species, are closely related and can interbreed. Astyanax jordani, however, is another blind cave fish, independently and recently evolved from the sighted surface form, which is sometimes confused with the cave form of A. mexicanus. However, when born, the cave dwelling form of A. mexicanus has eyes. As they grow older, skin just grows over them and the eyes degenerate completely, because there is no need for sight in the pitch-black world of a cave.