There's a mission early in the game called "Club Bam Bam". In the first fight inside the club, you can see some aquariums across the walls. Grab an enemy with and drag him over to do an environmental kill with an aquarium. A fish will fall out onto the floor. Pick it up with and start swinging. Holding for a heavy attack while holding the fish will result in a quick one hit kill. This fish has a very short life span, so try to KO one of the weaker Thugs.
NOTE: You must break the one on the right side. As that's the only one that drops a fish. If you do not get it on your first visit to the club. You can go back and replay the mission at a later time.
Here is a video guide by bs000.