You can get unlimited with Angelfish (The Bait) Also stocked ponds will let you fish all day 24/7 well not really because all sims need to sleep!
How do you come up with unlimited? The pond/river at the graveyard will only give so many Deathfish before it's either tapped out or dawn comes. You'll either run out of fish to catch or run out of time before you can get more than 6, to my knowledge. I always use Angelfish to catch them, too.