What Is the Slowest Fish?

Answer:

Sea horses (family Syngnathidae) are incapable of swimming against the current and, to avoid being swept away, hang on to coral and marine plants with their prehensile tails. Their swimming ability is severely limited by a rigid body structure. The major source of propulsion is the wave motion of the dorsal fin: this makes a ripple, which drives the fish forward in an erect posture.

Some of the smaller species such as the dwarf sea horse (Hippocampus zosterae), which reaches a maximum length of only 4.2 cm (1.7 in), probably never attain speeds of more than 0.016 km/h (0.001 mph).