***Potential Spoiler***

they were a clue... the bag they were in had sprung a leak so the person who had them put a nicatine patch to stop it, killing the fish. then in the dorm room later on mcgee notices the مچھلی tank is empty and points that out to gibbs... then when the killer is found, shes trying to quite smoking and has the nicatine patch... hope that was helpful=)