The Jackknife Fish, Equeus lanceolatus, Its elongated dorsal fins and curvaceous caudal fins often ripple in light water currents. The Jackknife Fish thrives in tanks with plenty of live rock work where they can hide. If multiple Jackknife Fish are desired, they should be added together to curb against any territorial aggression. Often during acclimation, the Jackknife Fish will hide themselves and not come out for days until they feel safe - this is completely normal.