That is a question I have asked myself sevral times but the answer sorta
depends on what level you are.
A few fishes that give much money from as little effort as possible is the Red-crow, Red Oscar, SeaBream, Garish Hind, Parrotfish, Spotted Parrotfish they're easy to catch just using normal bait and gives a few easy bucks. These fishes is around the 2k buck in SSS and at S witch is the most freq. catched giving about 1k per fish. (-50$ for type A bait.)
Atleast thats what I think using the rod I have and the level I'm at now.
Lvl40 Using a Hurricane Rod.
/Jon