Short answers, pulled from the story.
A fish is an aquatic, anamniotic, gill-bearing vertebrate with a tough cranium protecting the brain but lacking limbs with digits. Fish divide into the basal jawless fish and the more common jawed fish, which include living cartilaginous and bony fish plus the extinct placoderms and acanthodians. The study of fish is called ichthyology.
There are over 33,000 extant species of fish, easily the largest group of vertebrates and more numerous than all amphibians, reptiles, birds, and mammals combined. As of 2016 there were over 32,000 described bony fish, over 1,100 cartilaginous fish, and over 100 hagfish and lampreys. Teleosts make up 96% of all fish species.
Fish are paraphyletic because any clade containing all jawed fish or all bony fish also contains the tetrapods, the four-limbed vertebrates usually not counted as fish. For this reason the old class Pisces is no longer used in formal classification. Fishes of the World notes that tetrapods, including humans, are simply modified bony fishes.
The Devonian is called the age of fishes because fish diversity greatly increased during it, including among placoderms, lobe-finned fishes, and early sharks. The first jawed fish, the placoderms, appeared in the Silurian and diversified enormously in the Devonian. Bony fish became dominant after the end-Devonian extinction wiped out the placoderms.
The deepest fish found in the ocean is a cusk-eel, Abyssobrotula galatheae, recorded at the bottom of the Puerto Rico Trench at 8,370 metres. Fish inhabit nearly all aquatic environments, though none have been found in the deepest 25% of the ocean.
Fish show cognitive capacities including self-awareness in mirror tests, demonstrated by manta rays and wrasses checking their reflections. Choerodon wrasse, archerfish, and Atlantic cod can solve problems and invent tools, and fish can learn to traverse mazes using spatial memory. Behavioral research suggests fish are sentient and capable of experiencing pain.
Overfishing reduced the Atlantic northwest cod population to 1% of its historical level by 1992, and the Pacific sardine fishery off California fell from a 1937 peak of 800,000 tonnes to 24,000 tonnes by 1968. The Food and Agriculture Organization reported that in 2017-34 percent of the world's marine fish stocks were classified as overfished.
Fish carry symbolic significance across many religions, with fish offerings made to the gods in ancient Mesopotamia and fish serving as a major symbol of Enki, the god of water. Early Christians used the ichthys to represent Jesus, the Hindu deity Matsya took fish form, and the constellation Pisces traces to a Roman legend about two fishes rescuing Venus and Cupid.