As much as fish, plants and coral appreciate light some of the day, they're not overly fond of an artificial sun blasting into their tank around the clock. If you don't switch off your aquarium lights at night, you'll need to during the day, or big problems will arise.
You can shut off your aquarium's lights anytime you want, so long as they're consistently on for about 12 hours and off for the same length of time. Most aquarists prefer shutting them off at night because they would rather see the tank lit up during the day. If your work schedule keeps you away from home during the afternoon and evening and you're awake at night, you might prefer the opposite. If you do keep your lights on during the night, keep the light inside your house to a minimum during the day. If your blinds are open and light bulbs are shining into your tank, you're not achieving the equal day and night environment.
Just like any freshwater plants you have in your aquarium, algaes need light to flourish. If you leave your tank lights on all day and night, you're providing a haven for the organisms to smother your plants, decorations and glass with their greenness. While certain fish and snails eat algae, leaving your lights on 24 hours each day will likely lead to an outbreak that's too much for even them to handle. Additionally, certain types of algae -- notably blue-green algae -- are stubborn buggers that most algae-eaters don't eliminate. Blue-green algae can also adversely affect your aquarium inhabitants' health if the algae amass in large enough quantities.
Fish, Plant and Coral Health
Your fish have the best chance to thrive when you mimic their natural habitat. Assuming they don't have crazy rave parties in the wild when the sun goes down, that means fairly even amounts of light and darkness. Some fish species are diurnal and others are nocturnal. Both types need a switch between light and dark periods to stimulate their activity levels. Without that switch, the fish can become stressed and sick. Plants and especially coral also need a 12-hour on and 12-hour off cycle.
If you'd like to watch your fish swim around at night, you can opt to add a moonlight to your tank. A moonlight typically features several tiny LED bulbs that produce a blue ambient light throughout your tank. Moonlights don't affect the night and day cycle of your fish, coral or plants, and they don't contribute to algae growth.
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