They're at the unnamed beach that is up the road from Founder's Beach, which is located behind the Art Museum and the Library. You can find the spot they'll appear any time of day by looking for the fish swimming around the pylons that are left over from a ruined boardwalk. Between 5am and Midnight, there will be vampire fish and red herring swimming there. Between Midnight and 5am, a cloud of fog will appear over the spot, and there will be deathfish there.
Likewise, you do not even need to be level 10 fishing. At level 5 you can start using live bait, and pretty much catch any fish (it goes slower when you don't have the level, though). Use cheese to catch alley catfish, then use the alley catfish to catch angelfish. Once you have some angelfish, you can use them to catch deathfish at level 5. However, you won't catch many. Expect maybe about 4-5 if you start fishing right at midnight, and continue fishing until they disappear.