Do Fish Need Light?

The answer to how much light an aquarium needs is not straightforward. There are average numbers and rules of thumb available, but are they useful? In this article, you will learn if light is needed in an aquarium, what is blue light, and how much light your aquarium needs.

A fresh water aquarium with some aquatic plants and decorations with light streaming down from the hood.
Do fish need light to see how beautiful their aquarium is?

Do fishes like light or dark?

First and foremost, tropical fish alone don’t care about the amount of light that an aquarium light provides. Not that fish don’t need light. Fish have eyes of course and they may need artificial light to see their food and the other things in the tank. However, fish can comfortably live by the ambient light coming from the room (unless you live in complete darkness).

A basic rule of thumb is to have:

  • 1 to 2 watts of fluorescent bulbs per gallon for plant-less aquariums.
  • 2 to 5 watts per gallon if your tank has live plants.
  • Lamps should be left on for 8 to 12 hours per day.

Why is light necessary for aquarium fishes?

Light is beneficial to your fish. Fish have photoreceptors in their eyes and light can help them grow and produce their vibrant colors. Light can also help your fish grow.

The light in a fish tank is more critical to your aquarium plants so that they can photosynthesize. Mostly, it’s so you can view your finned friends. That means, if you have artificial plants, it doesn’t matter how intense the fluorescent lights or led light are, how many leds in the led aquarium light or fluorescent bulbs there are, and the spectrum of light is of little concern. Only if you have real plants do you have to consider these things. Some lamps are calibrated for the exact purpose to enhance the color of your tropical fish. Like making your betta fish or neon tetra really show off their vibrant colors. These type of lights perform less well for the photosynthesis of plants, but the colors in your tank will be vivid.

Do fish like blue light?

Blue light aquarium bulbs emit primarily light from that part of the light spectrum. It’s highly beneficial for your live aquatic plants. It mimics the moon light and if you have nocturnal fish, they’ll come out of hiding when you have the blue light on. Your diurnal fish will think it’s becoming night so they’ll rest and sleep. If you have nervous fish, the calming blue light will help them venture out since they think it’s turning into night.

A study on the effects of blue light on goldfish found that their eyes can be affected by blue light. In general, most aquarium owners don’t keep their blue lights on all night so this shouldn’t be a big concern.

How much light do fish need a day?

The good news is that you can still use whatever aquarium lights you have and leave them on for 8 to 10 hours. Leaving them on for more could promote algae growth. If the latter is a problem, you can reduce the amount of time the aquarium lights are on or replace part of water more frequently. If you have your tank near windows, the sunlight can also reduce the amount of artificial light your plants will need.

The above guideline (that you need 2 to 5 watts per gallon of water) is for fluorescent bulbs. They still dominate the market of aquarium lighting but are rapidly being replaced by LEDs. LED lamps are more energy efficient and emit the same amount of light while consuming much less electricity. Another benefit of LED aquarium lights are they also don’t produce as much heat as a fluorescent bulbs.

It’s easy to convert how many watts of LED aquarium light you need to receive an equal amount of light from fluorescent bulbs. A simple way to do it is to divide the amount of watt by four. That means you would need 0.5 to 1.25 watts per gallon of water if you use LED lights. But is it helpful? Only slightly.

How long should you leave the light on in a fish tank?

The required time to have your aquarium lighting on and the intensity of bulbs in your aquarium depends on several things.

  1. How long are the lights on? In general, a tank should have lights on for 8 to 12 hours. But it depends on how many watts per gallon that you need. Measuring light in watts per gallon is a weird method in our opinion but is something that seems to regularly used. A watt is an instant measure of power or the rate of energy consumption. To calculate the energy your fish tank receives, you need to multiply the watts with the number of hours the fish tank light is on. For example, if you have a 1-watt lamp and leave it on for 12 hours, your tank will receive the same amount of light it would receive from a 2-watt lamp left on for 6 hours.
  2. Does your aquarium get natural light from outside? If it does, how can you measure how much? Of course you do not place an aquarium in direct sunlight. Things to look as is does the amount of sunlight change throughout the day? Is it possible to take that into account? It may be possible, but we’ve found that it isn’t that easy. That said, we’ve found that aquariums that get sunlight can be fine with the above calculations. If you notice algae growth, reduce the amount of time you have your aquarium lighting on.
  3. What kind of plants will you have? The amount of light required by plants is not equal from plant to plant. Some plants love the shade and some need an immense amount of light. If your fish tank has poor lighting, it does not mean you should set up artificial plants only. You can easily ask for low light plants in a pet store that sells plants.
  4. How tall is the tank? The light shines upon the surface of the water. The taller the tank is, the less light gets down to the bottom of the aquarium. The rule of thumb above are useful only if you have a “regular” fish tank in a broad sense of the term.

Thankfully most fish tanks come with lights already installed in the hood. If you are just starting, we don’t recommend that you buy a tank without lights already installed in the hood. If you calculate the number of watts based on the above rule of thumb, you could quickly conclude that your tank needs more light. But does it? For example, our 16 gallon aquarium has only a single 15W fluorescent bulb. This means it gets approximately one watt per gallon. But that is not a problem because we leave the fluorescent light on for 12 hours and aquarium plants seem to thrive.

We think the rule of thumb is useful only if you have nothing else to start with. If you don’t have pre-installed lights, there is no limit on how large of a light fixture you can place at the top of the aquarium. You can start with the rule of thumb mentioned above to calculate the artificial light your tank needs.

After that, set your lighting system timer for an average amount of time, say 10 hours per day and see what’s going on. If your live aquarium plants seem to languish, increase the time and see if any changes occur.

You now know if light is needed in your aquarium, what blue light is, and how much light your aquarium needs. This will go a long way towards a happy life for you and your pet fish. And keeping your Pet Happy is what we all want.

Next in series: How to set up an aquarium.

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