Hath

The Hath were a race of piscine humanoids.

Appearance
Hath were a humanoid race with purple and orange, fish-like skin and small headcrests running along the top of their heads. They were slightly taller than most humanoids. They breathed nutrient liquid rather than air. When in a human-breathable atmosphere, they used a breathing structure attached to their mouths. (TV: The Doctor's Daughter)

Technology
They possessed Progenation machines and generation chambers like the humans they had been allied with. They used the machines for the same purpose as their enemies, to create new soldiers. (TV: The Doctor's Daughter)

The Hath used heavy duty blaster rifles that fired barrages of bullets and long plumes of flames from the guns' barrels. They had the same holographic mapping system as the humans and when the Tenth Doctor uncovered a suppressed layer of information it showed up on the Hath map as well. (TV: The Doctor's Daughter)

History
The Hath left their planet by themselves, but later joined up with other species like humanity to share technology. They helped mine the ores needed to create the first colonies. (TV: The Doctor's Daughter)

In 2010, two Hath were seen at an extra-terrestrial bar when the Tenth Doctor came to visit Jack Harkness one last time before his regeneration. (TV: The End of Time)

The Hath were one of the 11 creatures who appeared in the hologram projection shown by the Atraxi as they were scanning Earth's history to check if it was protected. (TV: The Eleventh Hour) This is particularly curious, as the Hath have never set foot on Earth. It is unknown how the Atraxi had access to that clip.

The Hath took part in the colonisation of Messaline in July 6012 of the New Byzantine Calendar. With the death of the mission commander, there was a power vacuum and the species fought to take the planet for their own.

After a seven-day battle, the two races found the Source, an artefact that mythology told them was the breath of the creator. Due to the efforts of the Tenth Doctor, Jenny, Donna Noble and Martha Jones (who had allied herself with the Hath) the Source was broken and terraforming gases within escaped. The two races ended their conflict and agreed on peace after the murder of Jenny by Cobb, the leader of the human forces. (TV: The Doctor's Daughter)

Behind the scenes

 * Russell T Davies' original concept for the Shadow Proclamation scene in The Stolen Earth included an appearance by several Hath, but his plans to feature a "rogues gallery" of alien races in the scene fell through; ultimately the Judoon were the only returning race featured.
 * Prior to the broadcast of The Doctor's Daughter, it was rumoured that the Hath had the appearance of red bubbles and communicated through expanding and letting out smaller bubbles.
 * While bubbles did appear to be the Hath's only form of communication, Doctor Who Confidential showed that lines were actually scripted for the Hath (even if they only groaned throughout the episode).
 * It has been suggested by the director of The Doctor's Daughter, Alice Troughton, that the fish seen in the Source Chamber are the ancestors of the Hath