i started playing this game yesterday and i havent had any luck, my fish just keep dying before they even become adults because they are unhealthy i dont understand how you get their health bar up
i need help!

You have to be careful about which fish you cross-breed at the beginning of the game, because many species of fish won't be able to survive until you've upgraded your tanks. This thread has links to information that will explain in more detail why fish lose health and die, and how to successfully get started in Fish Tycoon.

as one hint, most crossbreeds of two common fish with one characteristic in common will produce an offspring that will live long enough to become an adult.
For example, a greenfin spotanus and an orange spotanus. or a greenfin spotanus and a greenfin beta. where you run into trouble is breeding one of these offspring with one of the common fish. Usually that results in a species that you need higher environmental research to keep. Often you'll get a surviving fish with two offspring of common fish though.

I still think researching environment is the first thing to do. but you need money. A good high priced fish is the speckled leaffish. HERE are ways to make it with common fish. Breed a bunch of these and sell them for $75 - 110 (depending on how much advertising you have and such) (click link and scroll down a bit)