I've been trying to find this movie for a solid year or so now and the lack of ANY information on it makes me think I'm making this up. I saw it when I was young so 1990-1996 is the best I can do time frame wise.
From what I remember the movie is about a Scientist who splices his own DNA with that of a fish hes been experimenting with. I want to say it's a generically altered Zebra Fish but I'm not certain of that at all. It's your typical "creature hides in the shadows and kills his victims" kind of movie. The only other thing I remember about it is at the end someone brings in the same kind of generically altered fish in one of those "won it at the carnival" bags. Credits.