Go around town with your shovel and hit every rock you can find. One of them should turn red when you hit it. Keep hitting it to get more money
Every New Year your parents give you 10,000 bells. Change the system date on the Gamecube; or in the Animal Crossing options and change the date to the new year. Check the mail box and you will have 10,000 bells from your parents. Repeat the process, but change the year
Dig for fossils every time you play. When the Museum fills up with fossils, you can make a mint at Nook's or trade with residents for good items with fossils. Skulls of dinosaurs seem to be worth the most, usually bringing in 4,000 to 6,000.
Get three fossils, then send them to the museum. Then, change the day to one day later. Check your fossils. If they are rare, continue playing. If they are not, reset the game until you get rare fossils. You can either keep them for H.R.A. points, sell them to Nook, or donate them to the museum
Play the Stalk Market. Turnips do not seem like that much of a commodity, but its gold in the game. You can make a fortune buying Turnips and selling them at the right price
If you have at least 10,000 Bells, on Sunday Joan the turnip seller will at your town every from 6 a.m. to 12 noon. Buy 100 turnips as many times you can. The next day, go to Nook's store and sell them all. It is possible to get 73,600 Bells just for 100 turnips
Barred Knifejaw 5,000 Bells
Bass 300 Bells
Bitterling 1,300 Bells
Blue Gill 120 Bells
Brook Trout 150 Bells
Carp 300 Bells
Cherry Salmon 1,300 Bells
Crucian Carp 120 Bells
Dace 200 Bells
Freshwater Goby 300 Bells
Giant Snakehead 6,500 Bells
Gold Fish 1,300 Bells
Guppy 1,300 Bells
Koi: 2,000 Bells
L. Bass 3,000 Bells
L. Char 10,000 Bells
Piranha 6,500 Bells
Plae Chub 200 Bells
Pond Smelt 300 Bells
Popeye Gold Fish 1,300 Bells
Rainbow Trout 650 Bells
Red Snapper 3,000 Bells
S. Bass 200 Bells
Sea Bass 120 Bells