Where to find rare fish?

Solution 1:

There are rare fish in some of the fishing areas that can't be found anywhere else. The specific rare fish that I believe the character was talking about in that email would be the Twinfish and Bloatfish, as those can only be found near that character's house.

Other rare fish would be the Basking Shark and Swordfish (and their machine lifeform variants) near the Missile site in the Flooded City, the Coelacanth (and its machine lifeform variant) near the coast in the same area, the Beetle Fish and Arowana Machine in the Oasis, and the Fur Carp and Arapaima on the path to the Ravine.

Those fish can only be found in those specific locations and some of them are fish I've never even seen.

For the full list of fish in the game, here's a link to the wiki.