Fishing with the master rod definitely gets me tons of money like 18000 a day. Just go fishing and usually anything "large" will sell for like 500g.
I use this method as well. As I love wasting money on crops that I end up letting rot later on, I ended up making a little list of helpful fishing tips. So... enjoy said tips!
1. Fogu specifically says that rod fishing has nothing to do with the time of day. However, I've noticed that expensive fish do
happen to be more common at from 11 PM on, even though they can be obtained at any time.
2. One must be very careful where you are fishing sometimes. In spring, avoid using your normal fishing rod in Kono Mid W during sunny weather, as you will repeatedly catch the Dragon Carp, which will efficiently waste your time. The same goes for not using your normal rod in Kono High after midnight in fall, for you will catch many Giant Salamanders, and Blue High or Kono Mid S in Summer when it rains (excluding typhoons,) for you will catch multiple Giant Catfish.
3. Be extremely thoughtful about time in winter. Once you have caught all three other "King Fish," you won't get through a single fishing in Blue High and Kono High without catching at least 2 Giant Hutchens, which gets VERY annoying, especially since large trouts are very abundant in those two fishing areas.
4. Think about what season you are in when you fish. There is usually one fishing spot that is amazing and one that is useless in each season.
5. Always fish during storms, and fish in as many spots as you can, no matter whether you'll get a King Fish or not. The fish in typhoons especially are really
expensive compared to the regular fish.
If you don't feel like fishing, selling food gotten as request rewards, catching bugs, selling your repetitive cooking festival diamonds, finishing requests... it's really up to you how you make money. I prefer this "epic fishing," but everyone has their preferances.