Why does my tank water go murky quick?

You have about double the amount of fish 80 liters can handle (21 gallons) there is no way the filter that came with it can handle the bio load. I'm surprised you haven't started losing fish yet! If you can't bear to part with any fish, get another 80 liters tank right now and get it cycling! In the meantime, do partial water changes, about 25% every four days until the other tank cycles and you can move the excess over. I would move the barbs, shark and rainbows.