I have the difficulty set on easy, and my baby fish seem to die before they get very big. They are well fed, and I have the aeration system installed in the tanks. They don't show any signs of illness but their health bars are way low. An hour later, they're dead. What am I doing wrong?

At the beginning of the game, before you've researched higher levels of Fish Environment, you need to avoid breeding different species of fish. Doing so will create fragile hybrids, which can live only in better tank environments. Try to breed identical or nearly identical fish and sell them until you have enough money to improve your research level. There are some hybrid fish that sell for better prices and will live long enough in your original tank environment to breed and sell them, but you'll probably have a lot of dead babies during your search for them.

I noticed that with certain breeds and with the initial starting tank environment (Level One), many baby fish never made it past year 6 (give or take a year). Once I upgraded the tank environment to Level 2, many of those same babies (same parental combinations) still died, but now they made it to year 16 or 17. So clearly tank environment has a HUGE impact on the viability of breeds. Now I am working towards $1,500 in cash to upgrade to Level 3!