To be fair you just need to keep an eye on what is selling and what isn't. Plus there are pools with high value catches that have a narrow range of fish (4 at most) so that kind of helps.
For the record I'm pretty close to 100k profit this week from just fishing casually every night and the week isn't over.
Side note: to the person who seems to think that every catch is guaranteed.....you need to fish more. Let's have a nifty comparison.
Mining/Botany: "*CLUNK* "You got nothing" - onoes you missed and didn't get something you already didn't have.
Fishing: *WHIRRRRR, SNIP* "The fish took your lure" - welp, that's 2-4k you just lost.