Photoperiods in Nature
There are dramatic differences in the light cycles, or photoperiods, experienced by the many types of fish and other aquatic creatures that you might choose to keep. Animals living near the equator are adapted to fairly stable cycles of twelve hours of bright sunlight and twelve hours of darkness each day, while temperate species experience drastically changing amounts of daylight throughout the year.
The intensity of the light required by various animals is also a matter of great importance. Animals from shallow freshwaters or coral reefs will generally require very bright light, while nocturnal species or those living in sheltered waters may be quite stressed by even moderate amounts of light. As you have learned by examining other facets of your hobby, the best way to keep your animals properly is to learn as much as you can about their life histories.
Once you know your animals' light requirements, it is relatively easy to create the proper photoperiod. Light timers can be used to mimic a natural day-night cycle. It is also advisable to provide periods that correspond to dawn and dusk. This can be accomplished by setting a timer to allow a dim lamp to come on before the main lamps, and to remain on after they have been shut off. Alternatively, a room light can be timed to come on and go off an hour or so before and after the main aquarium lights. In addition to providing a more naturalistic environment for your pets, this system will ensure that sensitive animals will not be shocked by a bright light suddenly flooding a dark aquarium.