The cloud belt was a layer of crystalline fog in the atmosphere of a planet Elliot Sardick lived on. Sky fish were able to swim through the electricity in the fog. Human singing resonated the crystals in the fog and created delta waves, which attracted the fish.
As the larger fish were predators of the people who lived on the planet, Sardick created a machine that would allow him to control the cloud belt. He could release the sky fish into the air beneath as his whim dictated, giving him power over the populace.
One day, the cloud belt fell and the fish entered a school in Sardicktown. This incident was significant enough to have enter lasting memory and remain a primary topic of conversation for a long time after.
Elliot's son, Kazran Sardick, inherited the machine. Warnings were posted when fish were predicted to be passing down through the belt. Some believed this happened when Kazran was in a foul mood, prompting him to release the creatures. On one occasion, a ship carrying 4003 passengers entered the belt and was set to crash. The Eleventh Doctor attempted to persuade Kazran to release the ship from the belt, but he refused. The Doctor altered his past, attaching him emotionally to Abigail Pettigrew in an attempt to thaw his heart.
When Kazran eventually attempted to save the ship he found that the belt controls - being isomorphic - no longer responded to him, as his identity had changed too drastically as the Doctor altered his past. The ship was saved by Abigail's use of a song that resonated the particles in a manner that prevented its terminal descent. (TV: A Christmas Carol)