If you're looking to maximize your profits from fishing, the legendary legend is your ultimate target selling for a whopping 5000g and reaching 10000g with iridium quality but you can only catch it once when it's raining in spring at mountain lake (and i never sell them - only display them to my fish pond collection/aquariums). Something interesting to keep in mind, there are quite a few other valuable options worth pursuing too.

The crimsonfish brings in 1500g to 3000g and shows up near the pier during summer, while lava eels swimming around floor 100 or the volcano can net you 700g to 1400g. Down in the winter night market submarine, you might get lucky with a blobfish worth 500g to 1000g, and exploring the mines around floor 60 could lead you to an ice pip that'll sell for up to 2000g. The angler hanging out near the bridge by jojamart during fall can bring in 900g to 1800g, and sturgeons are great not just for selling but turning their roe into valuable caviar.

A good thing to remember is that with the new fish smoker and artisan profession your profits can actually double - like getting 5880g from a smoked iridium lava eel. And if you're into processing fish, caviar from sturgeon roe sells for a solid 500g making it worth the extra effort.