The Hath were a race of piscine humanoids and they are one of the villains in the Doctor Who episode called, "The Doctor's Daughter". They are desended from fish and breath a nutrient liquid. When they are not in the proper enviorment they use a breathing structure that is attached to their mouths.
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.
They fought a seven day war with the humans in the future over control of the Source. While separted from the Doctor and his colleges after an attack by the Hath, Martha Jones helped the Hath find the source and the Doctor, Donna, and Jenny assist the human in finding the Source as well. The two factions reach the Source at the same time but before they can fight the Source broke and the terraforming gases that were in it escape. After this the two sides stop fighting and agree on peace.
One of them later appeared in an alien bar, when the Doctor said goodbye to Captain Jack Harkness.
They are shown on a hologram by the Atraxi.