Pay Attention to Sales
If you notice your fish are selling super fast, consider raising the price a bit. Of course, if they're not selling at all, chances are they're priced too high and you need to lower the price so people will actually buy them. Remember that when you change one fish's price, it alters the price of all other fish of that species.
Speed Up Time
If you're tired of waiting for your fish to breed or grow, just speed up your computer's time clock a few of hours. Fish Tycoon's in-game clock corresponds to your computer's clock, so when you change the time, it changes in the game.
Making Money
Some of the first things you should do when you have the chance is to increase your aquarium size to Tank 2, so you can breed more fish. Also, research advertising to bring more customers to your store.