You probably have gold chromis or yellow chromis. Several different fish fit the discription you gave, and their common trade names frequently get mixed up. You may even have a black fin gold chromis *shrug*.
Fish get stressed from over-lighting the same way humans do. Fish need sleep, and the constant presence of light distrubs their sleep cycle just the same as people working overnight shifts find it difficult to sleep during the day time. Light triggers hormone production of the pineal gland in the brain, and this regulates things such as body temperature, migration urge, and sexual urges. The presence or absence of light can also create confusing stimulation in the brain as far as spatial orientantion and directional sense go.
While it is possible to acclimate to over or under light stimulation, it takes awhile and the process causes stress. The chronic stress can lower immune response and make an individual more subceptible to disease. It also has psychological repercussions.
A scientist (don't remember the name) did an experiment where he lived in a cave with no access to natural light or light cycles what so ever. He discovered that the human clock is actually a 25-30hr day, and when given no environmental cues, human behavior falls into that pattern of the longer day. It is sunrise, the actual presense of light on our eyelids being recieved by the brain that re-sets the pineal gland each morning, waking us up. The invention of artificial lighting has wreak havoc on the human system because each time we expose ourselves to light after being in a period of darkness (ie turning on a light when the sun went down) the pineal gland gets stimulated, thus leading to insommnia. Sitting in front of a computer screen at night in a dark room is essentially telling our bodies it is day time and we should be awake while our bodies really need to shut down and "regroup" with rest.
Fish have it the same way. They sleep. They need it to be dark in order to fall asleep. Light is bad, light means they are exposed to predation and must be on guard and wary, or else be eaten. You ever feel a sense of free floating anxiety? That's what fish experience when they cannot sleep or find safety. This can kill them.
~~Colesea
