You?ll have to go to the island between the times of 7pm and 5am, that?s when you?ll find rare beetles and sharks/big fish. You?ll need to be catching and selling those to make a lot of money, everytime I make an island trip and catch only rare beetles/sharks I come back with 300,000 to 500,000 bells worth of items. It could take a couple consistent hours of doing that however, depending on your spawn rates. Make sure you chase away any beetles on the inner circle trees (the fruit ones, etc), because rare beetles only spawn on the palm trees by the water.
But
only if you catch only sharks and rare beetles.
However, with catching sharks (if you haven?t already) you?ll need to hit the reel in button as soon as it bites down otherwise they?ll escape, there?s like a one second buffer zone with that.
With rare beetles, you?ll need to walk around your palm trees extremely slowly, running (and sometimes even walking normal speed) will make them fly away before you even have a chance to pull your net.
better yet, completely chop down any tree that isn?t a palm tree. this will greatly increase your chances of finding rare beetles.
same idea goes for sharks, you?ll be looking for their fin sticking out of the water, or a huge shadow. chase off any fish that don?t look like that. if your spawns start to lessen, go inside the island building and come back out. It seems to reset the spawns