No one is stabbing Slark in the back.

The tank doesn't hold any air but it makes a good back plate.
Now, it also carries precious water for Slark when he gets too far inland.



Part of the Slark's Diver Armor Set



Story:



After a small while in battle Slark realized that wearing fish does not offer much protection against a blade.
Looking for an alternative he remembered that when in prison, Slark sometimes saw Keen help out with the construction of the walls. He stalked Keen divers for days trying to find the right candidate.
Finally Slark convinced a Keen to give him the suit after cutting off the air hose. The plan did not work quite as planned and after a failed attempt by the Diver to reach the surface Slark got stuck with trying to pry open the thing to get the last occupant out. Few hours and a big hole in the face plate later, Slark finally acquired the Diver's Armor. That Keen smell will take a few days to get out.