It would definately kill the fish. The medication used kills all parasites - and I doubt it would distinguish a cold-blooded, water bound creature from a tape warm. Also, dewormers for cats and dogs are based on body weight. Cutting a pill in any size other than on a score mark negates the guarentee that you are getting the correct dosage of all chemicals in the pil.
Besides that, do you really think you could figure out how much a to give a one ounce fish when working with a pill for a 60 pound dog?
Never use animal meds except exactly as directed on the box, or by your vet!