If you do not have a lot of cash (especially in the beginning), sometimes buying medicine for a sick fish is not the best choice. If can recreate the breed with other fish in the tank, send the sick fish to the trash and make another.
Fish tanks can only hold 20 fish. If a pregnant fish has a baby when there are 20 fish in the tank, they will disappear.
What to Buy
Supplies are difficult to buy starting out. It gets easier once you find a breed that will make you $40+ for each sale. The first supply to work on buying is the Fish Environment. Without many baby fish will die.
Before you have researched Fish Environment, breed fish that look alike. You won't waste your time breeding fish that will die this way.
"Fish Tycoon" Breed Combinations
Take notes on what type of fish you get when you breed two fish. By keeping track of combinations it will be easier to find the magic fish. I keep a Word document with a table of the information. To see what two fish created a breed, put a fish in the isolation tank and click on the Species button.