Well, it worked the first time anyway. But the next time I tried to fish, the problem recurred. Now resetting the caches doesn't seem to do the trick.
I'm pretty sure the weird spot in Sunset Valley Park that I stocked (right next to the Sunset Valley Park Spot A, where Angelfish usually spawn) with one Goldfish is appearing exactly as such--it shows only Goldfish as available there. I don't think this matters one way or the other, but I guess it's possible.
I was hoping I didn't need to cheat or break the game, because I have gotten Deathfish without a problem in past saved games. But I guess that patch I've read about in other posts screwed it up and it was never fixed.