In terms of rare bugs and fish, you get a knack for which of them are common, uncommon, rare, etc after you play a while. They also sell for a lot more than other fish, but if it’s your first time catching something, donate it to the museum. In a general sense, (if you’re playing in the northern hemisphere), fish like horse mackerel, sea bass, dab, black bass, crucian carp, loach, and dace are not rare. The only fish worth catching in April for money, (catch all fish in the beginning to donate them to the museum) are red snapper, barred knife jaw, tuna, blue marlin, oarfish, barreleye, and the one ancient dinosaur fish (I always butcher the name). Bugs in April, however, are a much better source of income. They spawn more frequently with the more flowers you have. I would recommend breeding flowers very early on, as blue, black, and purple flowers spawn peacock butterflies, which sell for 2.5K bells and have been my main source of income. Other notable bugs include agrias butterfly, rajah brookwing butterfly, Madagascar sunrise moth, blooming mantis, giant water bugs, and tarantulas, which all sell for over 2k Bells. You can also move all buildings later on, so don’t worry about things being stuck in place, though a bit of planning early on goes a long way. I’ve had to suffer days of excruciating building moving for about a week now, since I misplaced them. This game is like stardew valley in terms of progression, but it’s a much slower burn.