This is a question which occurs often and the answer varies only slightly according to what is resident in your tank. Most photosynthetic creatures such as corals and plants have a photoperiod ( the optimum time in which they should be exposed to light) of around 7 to 8 hours and lighting for much longer than this is a waste of power as well as potentially harmful.

In general you should aim to reproduce the natural conditions of night and day; so darkness should prevail for around 12 hours per 24. Fish will need rather longer than 7 hours of light so if your tank is host to both fish and corals, aim for around 10 hours of lighting a day depending on the type of fish and coral.
The use of timers can be extremely beneficial as the periods will be un-varied which is good practice and also saves on the issue of forgetting to turn lights on or off.
How do I know which lights are the correct ones for my aquarium?
When you are looking at purchasing lighting for a new set-up for a fish-only tank, you should consider the capacity of the tank before you make your choice. A system with a total output of 1 to 2 watts per gallon is more than ample to ensure that your tank has enough light to imitate a natural cycle of light.
You can select the spectrum which most appeals to you because fish-only tanks are simpler to balance than coral or saltwater tanks and so you have a degree of freedom in your choices.
For freshwater planted aquariums you will need to consider the depth of your tank as well as the species of plants which you’ve added; usually a system with an output of around 2 to 5 watts a gallon will be plenty. There are a variety of “full spectrum” bulbs on the market which are excellent for promoting healthy plants.
If you have a saltwater tank with corals, then be aware that they require more power generally and will need between 4 to 8 watts a gallon. In the natural environment, most corals live at quite shallow depths of between 20 and 60 feet so they need plenty of light. However, some corals live at much lower depths and need much les light as they are not photosynthetic. Corals in the shallows which find that they are over exposed will grow in cracks and crevices in order to protect themselves from the elements…but in an aquarium you want them visible so balancing their light is a vital task if you’d like to have healthy and thriving corals.
If you choose corals which naturally require a low light level then the wattage can be eased up on to some degree and you can populate your aquarium with breeds of invertebrate which suit your needs too…so corals and invertebrates in salt water tanks will be fine with lower wattage as long as they are the breeds which thrive in dimmer light. This also means that you can turn off the lights after around 7 or 8 hours with no ill-effects.
Choosing lighting is fun and interesting; as you learn more about the inhabitants of your aquarium you will notice how quickly they respond to good conditions and also to conditions which are less than perfect…always ask for advice if you are uncertain.
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