what should i do when the health bar turns yellow?

The long-terms solution depends upon why the fish is losing health. If you have enough money, you can buy vitamins for the fish, which will improve their health temporarily. If the fish is sick, you need to cure it. If you're overfeeding the fishies, they will lose health, so try feeding them less often (wait until they are only half full on their food meter). Finally, if they are too fragile for the level of Fish Environment, save up enough money and research a higher level.

what can you do about the health and food meters dont move and the fish die

You can do what is suggested in the post above yours. Every reason for a yellow health bar is explained there.