Without cheating or using spawners, what you could do is take your sim to all the fishing spots, click on the water and choose "inspect water." That will give you all of the fish choices there. If there are fish that your sim has not caught or identified, it will probably say "unknown fish." Other than that, death fish normally appears in the graveyard. There are two graveyards in IP. The one you want is across the water from the science lab. It has a pond w/deathfish in it. Those appear from midnight til 4am.
Edit: you have to have lev 10 fishing in order to get deathfish.