Which small fish hangs upright in the water holding on to seaweed stems with its tail?

The fish you are referring to is likely the seahorse. Seahorses are unique for their upright swimming posture and often cling to seaweed or coral using their prehensile tails. This adaptation helps them stay stable in the water and provides camouflage from predators. They are also known for their distinctive reproductive role, with males carrying and nurturing the young in a special pouch.