Instant Money
Edit the Hex Code.
Editing the .pts file in the game is the ultimate cheat and provides instant money. In a hex editor (there are free ones available) use the following steps:
Open the fishsavegame1.pts (located in the Fish Tycoon folder)
Locate line 000003a0
Change each of the first three columns to the right to 1f
Save the file and open Fish Tycoon. You will now have 2 million plus dollars.
Use Time
If you don't want to change the hex code, the only way to make money in the game is to sell fish. Speed up this process by using the real time element of the game. From egg to selling usually takes about four hours. Breed your fish, or put new eggs in the water. Go to the Main menu and change the game to window mode so you can change the time on your computer clock. Move it forward three or four hours. Return to the game and sell the fish. Change your clock back, buy more eggs and repeat the process.
Sell High Dollar Fish
In the first batch of eggs, you will get the first 'magic' fish--the Greenfin Spotanus. You also start the game with enough money to buy two more batches of eggs. Buy them and drop them in the water along with the first batch. Use the time trick to mature all the fish. If you are lucky, you will end up with two Greenfin Spotanus. Now you have a cash cow. This fish is the 'fertility' fish and guarantees two babies instead of one. Priced at $55, they will sell like hotcakes. Sell every other fish, but keep this pair and breed them. Two will become six, etc. Use the time trick and breed them until you have a tank full (21.) Sell them--all but five or six. Repeat the process until you have lots of cash.
If you don't get a pair in your first three batches of eggs, you will likely get the second one in your next batches.