This cheat is quite easy to pull off and can make you a huge amount of money.

Alright then. First off: requirements-
You need to have a long line of coconut trees near the shore.
You will need a net.
You will need a fishing rod.

So far so good: now - wait until June/July/August (or travel through time but your neighbors will move and weeds will grow everywhere) and change the time to about 10:00pm.
Now go down to the beach and take out your net. If you whack the water with your net every time a little fish comes along you will scare it away. But when you find a massive fish with a fin that protrudes out of the water you´re in luck. Cast your fishing rod and try your hardest to catch it. If you catch it, take your net back out and try the whole thing agin.
Seeing as you have your net out, we can go bug catching at the same time. Ever wondered what those wierd coconut trees were for? They harvest four different kinds of rare bug that sell for a lot of money. When you´re trying to catch your large fish and you see one of these beetles, catch it.

When your pockets are full simply sell everything at Nooks store (Nooks Cranny, Nook ´n´ go, Nookway, Nookingtons) and then try the whole thing again. You can make about 100,000 bells in the space of 15 minutes.

Happy fish catching/bug hunting. Don´t forget to rate this cheat.