A breathing tube or breathing device, as the name suggests, was a structure which was designed to assist with respiration. These were often used by species who were living or working among other beings with different environmental requirements for breathing mixtures.

Barzans had to wear breathing devices when they leave their homeworld Barzan II, the device provided them with the gases that were found in the atmosphere of their homeworld and that were toxic to other species. (TNG: "The Price")

The Benzenites were known to have breathing tubes connected to their heads; if the tubes were removed, the Benzenite individual would die. (DS9: "Apocalypse Rising")

According to the Star Trek Encyclopedia, Benzenites and Benzites are the same species, which would be consistent with both using breathing tubes.