Exceptional tolerance to low pH of some tropical blackwater fish

Cardinal, neon, and emperor tetras (Cheirodon axelrodi, Hyphessobrycon innesi, Nematobrycon palmeri), small South American characin fish often found in acidic blackwater rivers, have an unusual tolerance to sulfuric acid solutions of low pH. Most cardinal tetras withstand immediate transfer to water at pH 3.5 for an indefinite period. With gradual adjustment to further stepwise lowering of the pH, one individual survived at a pH of 3.1 for five weeks. This is by far the most acidic solution known to be tolerated for extended periods by any fish. The lower incipient lethal level (for 50% survival) is estimated to be about 3.35 for neon and cardinal tetras. On the other hand, common guppies (Poecilia reticulata), a species generally considered to be highly resistant to environmental change, all died after 11 days at pH 4.75. Both guppies and tetras lose large amounts of body sodium on exposure to lethal pH's.