To start a new "Fish Tycoon" game, go to the Menu page. Click on Change Player. From here you can delete your old player file. There's sense in keeping it if you can't get any further. Then create a New Player.
"Fish Tycoon" Tips: Make Sure You Fish Don't All Die Again
If you are going to be away from the computer for a long time and cannot check on your fish to feed them, stop the time. From the Menu screen, click on Options. Select Stop Time under Game Speed. When you are ready to play "Fish Tycoon" go back to the menu and set the time back.
Some fish simply cannot survive your in tank if you haven't researched the required environment for them to live. Grow slowly from the first fish you have. Sell extra fish and save your money to research the second level of environment.