Declining health in the offspring of fish that you breed at the beginning of the game is a common problem for people who are playing Fish Tycoon for the first time. Although the description for Fish Environment says that it "allows rarer fish to survive and thrive in your tanks," it's not very clear that only the common fish types will thrive in a level one tank environment. You can see tips, by clicking on the posters in the Fish Store, that deal with that issue.

Hybrid species in a Level 1 Fish Environment will typically continue to lose health until they die. The only thing that will restore their health (temporarily) is Fish Vitamins, but their cost is usually much more than the fish is worth.

The best approach is to discover fish that do well in the starting tank environment and sell for more money than most of your starting fish. There are several fish that do reasonably well in your starting tank environment and are good money-makers (selling for $25-50 or more). As soon as you have enough money, start researching better Fish Environment. You'll soon be able to successfully breed the more fragile hybrids.

The selling tank can be used to breed and raise fish. Just be careful that you don't accidentally sell a fish that you wanted to keep. The only real difference between the selling tank and the other two tanks is that any adult fish in the selling tank will be "for sale" when you enter the Fish Store by clicking on the "Sell Fish" button.