To answer the riddle, a fish with no eyes is a "fsh." However, one fish species, commonly known as the blind cave fish or Mexican tetra, does lack eyes. These fish are born with eyes, which atrophy as the fish matures.
Though completely blind, the cave fish can easily swim through its surroundings, and researchers believe it is due to a sonar-like, internal navigation system. Thus, despite being at a seeming disadvantage, these fish have few special needs when raised in captivity. In fact, they make excellent single pets or additions to community tanks, because they are active and have a peaceful demeanor.