What You Should Know About Aquarium Lighting

Give Your Fish and Aquatic Plants the Right Amount of Light

illustration of how much light an aquarium needs chart

Illustration: Theresa Chiechi. © The Spruce, 2018

Aquarium lighting is both a design feature and a practical necessity. The soft inner illumination of a lighted aquarium makes it an important element of room decor. And both the fish and the live plants in your aquarium require light in order to thrive. But determining how much light the fish and plants need, and controlling that amount, can be tricky. In general, most aquariums require 8 to 12 hours of light each day (10 hours is a good starting point), at least some of which needs to be provided by supplemental aquarium lights. But this is a large range, and determining just how long your aquarium lights need to be turned on each day depends on a number of factors, including:

Light Needs for Live Aquatic Plants

A primary reason for using supplemental aquarium lighting is to provide the light necessary for living aquatic plants to perform their necessary photosynthesis. Just like land-based plants, aquatic plants use light to synthesize food from carbon dioxide and water, and they obtain this light both from ambient room light and from supplemental aquarium lighting. 

If there are live plants in the aquarium, the supplemental lighting period will probably need to be maximized. Plants require as much as 12 hours of light per day, but the precise length of time will depend on the aquarium setup and species of plants. If, on the other hand, your tank has no living aquatic plants, your lighting needs will be less—only what's necessary for the fish.

It is usually recommended that the aquatic plants in your aquarium be matched to the type of fish you are keeping: tropical plants for tanks with tropical fish; temperate cold-water plants for tanks with cold-water species. So light levels should also be chosen to match the natural inclination of the plants.

Tropical plants that will thrive on roughly 12 hours of light each day, all year long, include the banana, Amazon, Java, and fern plants. Cold water plants that do best with seasonal variation in light levels include the Anubias species and tiger lotus.

When you introduce new live plants to your aquarium, it is best to leave the light on for longer periods of time; this gives the plants a better chance to take root and grow vigorously as they're getting established.

Ambient Room Lighting

Just how long you'll need to run your aquarium lights will depend in large part on how much ambient light is already present in the room. It's even possible that you'll need no additional lighting at all, if, for example, your aquarium is in a bright south-facing sunroom with lots of windows or an active family room where ceiling lights and lamps are on most of the time. Ambient room lighting is often rather indirect, though, and chances are good that no matter how much light already is present in the room, you'll need to run aquarium lights for at least a few hours each day. But a room with good ambient lighting won't need the full 8 to 12 hours of illuminated aquarium lights.

It is also good to leave the aquarium lighting on longer in the winter, when natural light is less due to the season, for best growth and the healthiest aquarium plants. As the days lengthen, the aquarium lighting period can be shortened. 

Light Needs for Different Fish Species

When it comes to the fish themselves, aquarium lighting is mostly about you, not them. Lighting in an aquarium tank makes it easier for you to see and enjoy your fish, but it usually doesn't affect the fish all that much. The nature of an aquarium, with its four glass sides and relatively small size, means that most fish are getting more light than they do in natural settings, whether or not you are using supplemental lights. Most fish don't mind getting more light. A few species, such as cichlids and tetras, thrive on less light, and for these, too much supplemental aquarium lighting may affect them negatively.

Consider the conditions that a species experiences in the wild when determining how much extra lighting, if any, is required. Tropical fish have evolved under conditions that provided roughly 12 hours of light each day, so logic suggests that an aquarium with tropical fish will likely need a combination of ambient and aquarium lighting for roughly half the day. On the other hand, cold-water species such as goldfish, minnows, ricefish, and danios (zebrafish) are from temperate climate zones where the daylight hours vary according to the season. Here, you might want to vary the amount of light over the course of the year. To create a more natural environment, match the length of aquarium lighting to what the species experiences in its native environment. 

Light and Algae Levels

If excess algae is a problem in the tank, a contributing factor is usually too much light. Algae are tiny plants, and too much light causes too much algae growth. Reducing the time the aquarium lights are on to eight hours, or a bit less if necessary, will help reduce the algae growth.

Monitoring algae levels can, therefore, help you determine if your lighting levels are appropriate. If you begin to see excessive algae, shorten the periods of light to retard the algae growth. But remember that it's also possible to have too little algae in an aquarium. Algae are a food source for some fish species, so you don't want to eliminate algae altogether—just control it. 

Direct sunlight tends to create more algae than does artificial light. An aquarium near a sunny window may require less supplemental lighting than one on an interior wall. 

Controlling Lighting in Your Aquarium

The biggest obstacle to maintaining a uniform period of aquarium lighting is that owners find it difficult to turn the lights on and off at the same time each day. Fortunately, there is an inexpensive and easy way to remedy that problem.

Purchase an inexpensive on-off timer and plug the lighting unit into it. Set the on and off times to obtain the desired period of light. On-off timers are highly recommended for all aquarium owners.

Aquarium Lights and Heat

Remember that aquarium lights don't only produce light—many of them produce heat, and sometimes a lot of it. LIghting types that produce heat include incandescent, VHO-fluorescent, and metal halide. In smaller aquariums, these types can cause a significant rise in water temperature, sometimes enough to kill your fish and plants. If you use one of these types of lights, make sure to monitor the water temperature constantly, and avoid leaving the lights on overnight. 

Standard fluorescent lights produce cooler light and are a better choice for most aquariums. You can leave them on for long periods without danger, and many tropical fish and plants thrive under fluorescent lighting. 

Tips for Using Aquatic Plants

True aquatic plants must be kept wet at all times. If they are even allowed to partially dry when transplanting new clumps, these plants may suffer for weeks, or even die altogether. And live plants should never be removed and cleaned under running water. For true aquatic plants, such treatment will damage or even kill the plants. Some aquarium owners do this because they want to remove whitish slime on plants, but this slime layer is actually good bacteria. Fish eat this slime from time to time, and it is entirely natural—part of what keeps an aquarium in homeostasis.

To get the best looks out of your plants, plant them in the substrate and ensure they are well anchored to the bottom of the aquarium. Planting in thickets (bunches or clumps) is an eye-catching style, but don't use too many plants in a single thicket. The plants need room to grow and get full light on all the branches and leaves, which won't be possible if the thicket is too dense. Ideally, your fish should be able to swim cleanly around and through your aquarium plants.

Many aquarium plants will increase their numbers naturally. Vallisneria and similar plants send out runners under the substrate, which in turn then sprout upward next to the parent plant. These new plants can be left to grow where they sprouted, or you can cut away the runners and replant them elsewhere in the aquarium to start a new thicket.