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There is no doubt light is crucial for both plants and fish in aquariums, whether warm or cold water.
But, how long should you leave the lights on?
Frankly, there is no simple answer to this question since there are many variables to consider, and only a person telling you as much is being truthful.
That said, assuming everything remains constant, you will need 10 hours of light daily for an average aquarium.
However, tropical fish and plants, do better with 12 light hours a day, while cold water fish and plants will thrive with just 8 light hours daily.
Of course, if you only have fish and no plants in the aquarium, less light is required.
To better understand your aquarium light needs, let’s dig a little deeper, even look at a few fixtures that offer adequate full spectrum lighting inline with your tank needs.
Aquarium Light Schedule
To start with, you want an aquarium light schedule to help you keep tabs of your light timings.
Conventionally, your plan should include the hours and time of day you want your lights to run, and a timer like this Nicrew unit to help you monitor your plan rest you forget.
Then simply set the timer to turn the lights off at the same time each evening and to turn them back on the next morning.
For instance, I’ve scheduled my planted tank light to run almost all day.
It includes this Nicrew LED Gen 2 light that remains 10 hours daily, which is enough for my fish to feed and plants to thrive, though I only grow low-light plant species, though the light will work even for high-light plants.
This LED fixture should give off a dark blue light that mimick moonlight, which will adequately illuminate the aquarium even at night without disrupting the fish or plants night cycle. And is equally useful when you have visitors over at night.
That said, there are a few things to consider before setting up your light schedule. So, below are insightful answers you’ll find useful prior to starting:
Does Your Fish Tank Need Light
Well, if you are going to enjoy the aesthetics of your fish in the aquarium, you will definitely need light. Not only that, your fish need the light to feed and engage in other fishy activities, but also a few hours of darkness to relax and unwind.
Moreover, some fish species like goldfish may fade in the absence of adequate light.
Having said that, one important thing to keep in mind is different freshwater fish will have varying light needs.
Top and mid-tank dwelling fish will benefit from light better than bottom dwellers, such as plecos, catfish, and loaches, which are accustomed to living at the base of water bodies where light is scarce.
Fishys native to the tropics will perhaps also enjoy more light than those from subtropical regions where the sun does not penetrate the surface of the water as much.
Tropical Freshwater Fish Light Needs
Most common tropical fish come from rivers, streams and clear lakes in warm regions and are used to bright sunlight and warming rays in the natural 12-hours cycle.
Therefore, to best recreate these conditions in your aquarium, provide 12 hours of light a day, preferably, with bright LEDs that penetrate the water surface or make use of sunlight when you have a well-lit aquarium room..
However, some species of tropical freshwater fish like tetras may prefer low light. In which case, you leave enough hiding spaces in your tank for these fish to get away.
Cold Water Fish Light
Normally, cold water fish don’t have to be overly illuminated, only when you need to enjoy your aquarium, so anywhere in the region of 8hours of light is adequate.
However, if you’ve got enough hiding spaces for them, even 12 light hours are Ok, especially when your tanks hold light-hungry plants too.
To create the optimal environment for cold water species, try to match your aquarium lighting to seasonal daylight hours akin to those in the temperate region where most cold water species come from.
Top and Medium Dwelling Aqaurium Fish Light Needs
As we saw above, top and mid-level dwelling fish are used to (and prefer) more and brighter light exposure than bottom-dwelling fish.
A light period anywhere from 10 to 12 hours a day should be enough, but ensure the light is not obstructed and adequately penetrates the water surface.
Therefore, when keeping most of these fish, you do not want too many floating plants forming a canopy at the surface of your tank.
Bottom Dwelling Aqaurium Fish Light Needs
Bottom-dwelling fish, unlike their top and mid-level cousins, do not need as many light hours. Anywhere from 8 hours daily should be sufficient.
A canopy of floating plants at the water surface is also recommended, especially if you have bright light or a wide-shallow tank.
You will also note that most bottom-dwelling species prefer to feed at night but spend most of the day hiding. So, add a lot of hiding spaces in your tank, and preferably, allow more light for your bottom-dwellers at night.
Do Aquarium Fish Need Light During The Day
Yes, most tropical aquarium fish need light during the day since they are used to the daily light cycle with 12 hours of light followed by a period of darkness.
However, shy bottom-dwelling fish will prefer dimmer light, both at night and during the day. They are nocturnal.
Breeding females, sick or stressed fish also seem to like a lesser light during the day, perhaps to get the much-needed rest.
One way to ensure all your fish enjoy their preferred light levels during the day is adding hiding places in the tank using plants, driftwood, and other decorations.
Do Aquarium Fish Need Sunlight
Tropical aquarium fish do not need sunlight per se. However, placing your fish tank in an area with daylight exposure will help save on your lighting bill during the day.
Moreover, if you have plants in your aquarium, they will benefit from sunlight, which is more natural than any aquarium light fixture.
If you decide to allow sunlight in your fish tank during the day, keep in mind the duration should not be too long (more than 10 hours) because it will promote algae growth.
Too much light exposure will also increase the temperature in your fish tank, which might help save on power bills, but can be challenging to control because you can’t turn it off as you would do an aquarium heater…
…and moving a fish tank full of water, plants, decorations, and fish away from the sun is not an easy fit.
Light Needs for Live Aquatic Plants
While light in fish tanks is partly for your viewing pleasure, plants depend on diurnal light patterns a lot, and good quality light is paramount. (See how to choose planted aquarium lights).
However, the light used in your aquarium and the duration the light stay on will depend on the plants you have.
Some plants will thrive in low lights while others need high light for longer. Low light plants like Java moss, Java fern, and Anubias species will be fine with 8 to 10 hours of low-intensity light a day and the plants are perfect for beginners.
On the other hand, light hungry species prefer longer light hours, somewhere in the region of 12 hours a day, CO2 addition and fertilizer dosing.
That said, I also recommend you match your aquatic plants to the type of fish you have in your tank; tropical plants with tropical fish.
Ideally, keep platies, guppies, betta and cory catfish in aquariums planted with Java fern, Java moss anubias, hornwort, and Cryptocoryne.
Goldfish, minnows, ricefish, and Zebra danios come from temperate zones where daylight hours fluctuate, and are best paired with Anubias species and tiger lotus.
Light and Algae Growth
The most prominent con of keeping your lights on for long is algae. Usually, if you let the lights in your aquarium to run for more than 12 hours a day, blue-green algae will establish exponentially.
Although it’s not just your aquarium light you should be concerned about. but also natural light from the sun. Thus, when you have your tank in a brightly-illuminated room, placing it too close to the window is not recommended.
That said, algae will probably grow in your aquarium anyway, whether too much light or not, so what is important is to keep the extent in check.
In case of algae thriving in your tank even in fairly low light, consider adding algae eaters siamese algae eaters, shrimp like ghost and Amano, Amano shrimp, and snails (mystery, nerite).
Should You Leave Aquariums Lights on at Nights
Proper aquarium lighting is essential to fish and plants, but so is the period of darkness for them to rest and replenish. Thus, it’s best to turn off your aquarium lights at night to mimic plants and fish natural light cycle.
In case you weren’t aware, fish do sleep, however, most species don’t have eyelids, and depend on you to switch off the lights and provide enough hours of pure darkness for them to catch a nap.
Keep your aquarium light on for the recommended 8 to 12 hours then turn off the light to mimic the natural day and night cycle.
In case your fish gets restless when the lights go out, turn off the room overhead light an hour before your tank lights. This should give the fish eyes an hour to adjust to the lower light setting before complete darkness.
Do Aquarium Fish Need Light at Night
As I stated in the paragraph above, most tropical aquarium fish only need a maximum of 12 hours of light a day, followed by a period of darkness. Preferably, this should follow the daily light cycle.
As such, most of your fishes may probably not need light at night unless they are nocturnal bottom-dwelling fish or inverts like snails that prefer to feed at night.
For purposes of your aquarium aesthetics, you can use moonlight aquarium bulbs to illuminate your tank at night. The calm lighting will accentuate your aquarium but still allow your finny friends to get some sleep.
Please see this post (Do aquarium Fish Need Light at Night) for more insight.
How to Light Your Aquarium While Away on Vacation
Most people will remember to feed their fish when leaving for vacation but fewer remember to schedule their aquarium lights for when they are away.
While some aquarists are not even sure whether to leave the lights on or off while they have a good time away from home.
Frankly, fish in your aquarium doesn’t need light to live, hence leaving the lights off is fine. If anything, leaving the lights on could be a fire hazard and will encourage algae growth.
But if you have plants in your aquarium, then you will need to leave the lights on with a timer (recommended) running a diurnal setting.
Alternatively, if your aquarium room is properly illuminated, leave the tank near a window to take advantage of the natural day-night cycle.
Heat Emitting Lights and Aquarium Temperature
This is most likely not the first thing to cross your mind, but apart from light, aquarium fixtures produce heat, meaning you’ll have to consider that when choosing the light fixtures and bulbs.
This is especially true for smaller aquariums where the possible increase in temperature can be significant.
Generally, metal halides and incadescent bulb produce a lot of heat and are therefore not the best for aquariums. Besides, incadescent technology is pretty much outdated.
The more obvious and appropriate light source would, therefore, be either LED or fluorescent, but given the two, LEDs are the harder hitter.
Consider full spectrum LED rated for both aquarium plants and fish, preferably with night mode.
A thing to note if you choose to go with fluorescent; T5 or T8 fluorescent lights are not synonymous with VHO-fluorescent, which like metal halides, will spike your aquarium water temperature.
Also, T5 bulbs are preferable (as opposed to T8 and T12) because of their skinny bulbs, narrow footprint, availability, efficiency, and low heat output.
My Two Cents
Each aquarium environment is different, thus the only sure way to know how long to keep aquarium light on is trying out different setting until you find the perfect fit.
Even so, consider variables like natural and ambient lighting since every home is inherently unique.
First, think of your plants before the fish since fish tanks are less fussy about lighting. If anything, lighting a fish tanks is mostly for your own amusement.
Then do a little due diligence before buying or replacing aquarium grow lights, compare different products in the market to make sure you get the best bargain.
Moreover, since the question is how long your lights should remain on, and we’ve established the solution is a good light schedule, seriously consider a programmable light fixture with a timer, or maybe LED night mode lights.
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Enjoy Your Aquarium.