There really aren't 'levels' of health represented by the meter, despite the fact that the meters are color-coded. It's better to think of it as a percentage display - full being 100% healthy and empty being 0% healthy (and on death's doorstep) - with different conditions producing differing amounts of change in the fish's health. For example, overfeeding a fish reduces its health immediately by 2%. Overfeed it 50 times in rapid succession and, well, you know what will happen!