, the shark, pleco, and knife should all be re-homed. Assuming the pleco is common, he can reach 2 feet. If you wanted to keep all your current stock, I personally would recommend at least a 125g tank. But, you could rehome a few fish and get some smaller guys will do much better in a 38g tank.
The tiger barbs would probably like a few more friends anyway so they have a full school.
Is your loach a kuhli, black kuhli, or dojo loach? Those are the eel-like ones I know of off the top of my head.