It might just be a matter of patience - based on what I've seen in a game I'm playing right now, the more fish you catch, the better fish you catch, even after you supermax the skill.
If you have ten deathfish handy, it might be useful to build a small pond on your own property and use those fish to stock it so you're not limited to fishing for deathfish just late at night. It doesn't matter what quality the fish are - you'll be able to catch perfect fish from it eventually.