Should I Keep My Aquarium Lights On?
Should I keep my aquarium lights on during the day or during the night
Whether or not to leave the lights on is a question which depends on the type of fish - and plants - you're keeping.
As a rule, it's a bad idea to leave lights on overnight. Fish, just like people, need their sleep - not all of them close their eyes, but they all need to zone out for part of the day in order to stay healthy. For most popular aquarium species, too much light can mean not enough sleep. Different kinds of fish have different sleep cycles, and some are nocturnal. Aquarists with nocturnal fish are often tempted to turn the lights on at night so they can see them properly, but this will usually leave them panicking and hiding immediately - it the long term it can create dangerous levels of stress.
If you want to be able to see your fish after dark, you can get a special night light to make it possible. Using different frequencies of light and emitting a much gentler glow, this won't bother your fish. If you have coral in your tank, it may cause some of it to glow, creating a beautiful and distinctive night-time look.
Though some aquarists prefer not to use lights at all, it's generally a good idea to keep one on during the day, as this will help your plants to grow. Unfortunately it will help algae too - another reason not to leave it on all night as well!
Unless all your fish are equatorial in origin, it's worth adjusting the time you turn your light on and off in accordance with seasonal changes in daylight over the course of the year. Many fish have annual as well as daily cycles and this will help them to stay healthy. In species which have seasonal breeding habits, it can let them know when it's the right time to initiate mating behaviour.
Fish are very quick to associate the turning on and off of lights with other aspects of their daily ritual, such as feeding times and the times when humans (and other pets) are most likely to be close to their tank. Regular rituals are helpful in maintaining a low stress environment - so much so that some aquarists use electronic times to manage the lights when they can't be there in person.