How Many Watts of Light Per Gallon for Your Aquarium

By Eddie Waithaka @aquariawise

Aquarium enthusiasts often ponder the best ways to light their tanks.

Some think that more watts of light are always better, but this is not necessarily the case. To find the best lighting for your aquarium plants and fish, you must first understand how watts of light translate to gallons of water…

and…

With the advent of LED aquarium lights, you’ll also need to know how many lumens of light you will need per gallon of water.

Notably,

So, how many watts per gallon of water is ideal for your fish tank?

Well,…

1 to 2 watts of light per gallon of water is recommended for a freshwater, fish-only aquarium or a low-light planted tank. 2 to 4 watts per gallon is best for a medium-light planted tank, and 4 watts per gallon is ideal for an aquarium with high-light plants.

0.25 to 0.5 watts of light (10 to 20 lumens) per liter of water is recommended for a freshwater, fish-only aquarium, or a low-light planted tank. 0.5 to 1 watts per gallon (20 to 40 lumens) is best for a medium-light planted tank, and 1 watt per liter (40 lumens) is ideal for an aquarium with high-light plants. About 160 lumens per gallon.

That said…

Please note that aquarium light wattage and lumens per gallon only serve as guides. Other factors, such as fish tank depth, water clarity, and the height of light from the tank, matter when considering aquarium lighting.

So, in the rest of this post, we will explore the different aquarium light insights and help you decide which one is best for your aquarium.

Watts of Light Per X Gallon Fish Tank (Aquarium)

As noted above, you need between 1 and 4 watts of lights per gallon for your fish tank, depending on the plants and livestock you have. So, if you have a 10-gallon aquarium, you’ll require 10 to 50 watts from your fixture(s) in total.

For a 20-gallon fish tank, you will need about 25 to 30 watts for a fish-only aquarium and 30 to 80 watts for a planted aquarium, depending on the species of aquatic plants you have in the tank.

How Many Lumens Do You Need for X Gallons Planted Tank

Lumens per gallon can typically used to determine the appropriate lighting levels for planted aquariums. However, it is not commonly used as a standard because it is not specific enough to account for different types of plants, fish, and other inhabitants that may require light in a fish tank.

Essentially…

Even watts per gallon is not a standard measure because it only tells you how much power your fish tank bulb needs to produce light enough to illuminate your aquarium but nothing about the quality of light for your plants and fish.

But before the advent of LED aquarium lights, it was the only way (watts per gallon) to know which T5 or T8 light is ideal for your fish tank.

But with the advancement of aquarium lighting, including LEDs…

The more common and accurate measure (than lumens or watts per gallon), specifically for planted aquariums, is PAR (Photosynthetically Active Radiation).

What is PAR in Planted Aquariums

PAR (Photosynthetically Active Radiation) measures the amount of light in the visible spectrum that is available for photosynthesis. In aquariums, PAR values determine the amount of light needed for different aquatic plants to grow and help owners know which fish tank light to purchase.

Generally, 30 to 60 PAR measured at the substrate is good and adequate for growing just about every aquarium plant. Low-light planted aquarium plants require 10 to 30 PAR, medium-light plants 30 to 60, and high-light plants 60+ PAR.

To measure PAR in an aquarium, you can use a specialized light meter called a PAR meter. These meters typically have a sensor that measures the photons in the PAR spectral range that fall onto a plant each second, and provide readings in units of photosynthetic photon flux density (PPFD).

Now,…

When using a PAR meter in your fish tank, you will want to prioritize the reading at the substrate because it’s where the most of your aquatic plants grow.

This is not to say that the PAR readings at the top of your fish tank are not necessary because you might have floating plants, like moss or live driftwood branches, but light is often deficient at the bottom and side of a fish tank, depending on angle and height of bulbs, hence the priority for the reading at the substrate.

What is Considered High Light for Aquarium Plants

The number of watts of light per gallon and PAR for an aquarium depends on the type of plants you have and the size of the fish tank.

High-light for aquarium plants means a light requirement of PAR 60+ or 3 to 4 watts per gallon of water. This level of light requirement is typically provided by metal halide, T5 fluorescent, and LED lights and is necessary for the growth of most high-light aquatic plants. Granted, plants such as Sagittaria, cabomba, Lilaeopsis, and rotala need 80 watts of light or more to thrive in a 20-gallon tank.

Medium-light means a light requirement of PAR 30 to 40 or 2 to 4 watts per gallon, and low-light plants need PAR 15 to 30 and 1 to 2 watts per gallon.

Please note too much light will cause black beard algae even with high-light plants in your tank. Besides, most high-light plants can grow with a lower light requirement with CO2 injection, so you may not need to reach PAR 60+ in your fish tank.

High-light plants typically require more CO2 and nutrients to support their growth and will benefit from regular fertilization.

How Do You Calculate Aquarium Light Capacity

There several factors to consider when calculating the amount of light needed for your aquarium, including the size of the tank, the type of plants or fish you have, water clarity and the desired aesthetic.

You also have to consider the height at which you will hang your aquarium light and the configuration. If you plan to mount your light from the ceiling, you need more powerful bulbs than when you attach your light to a canopy closer to the water level.

That said,…

A standard method for calculating the light needed for a planted aquarium is to use a rule of thumb of 2 to 3 watts of full-spectrum lighting per gallon of water. 6400 to 6500 lumens is also considered ideal fish, aquarium plants and aesthetics, although these parameters do not directly translate to the amount and quality of light for a fish tank.

A better (more accurate way) too measure you aquarium light need is in terms of PAR(Photosynthetically Active Radiation), which is more reliable than watts per gallon or lumens.

A PAR meter will tell you the intensity and quality of light penetrating your aquarium water measured in micromoles per square meter per second (µmol/m²/s).

You will be able to know how much light is getting to your fish tank, right from the top to the substrate and even on sides and corners where light rarely gets to.

As noted before…

It is recommended to provide 15 to 50 micromoles per square meter per second (µmol/m²/s) for most aquarium plants to thrive, depending on species, stage of growth, light requirements, and CO2 injection..

How Do You Know If Your Aquarium Plants are Getting Enough Light

You can tell your aquarium has enough light by observing your plants or using a PAR meter to measure light density in different parts of your tank and compare that to your plant’s light requirement.

Aquarium plants that are getting enough light will have a vibrant green color. They will also develop quickly with healthy, lush foliage, while plants not getting enough light will have a pale yellow color.

If your plants are slow or stunted growth, with fading leaves, you do not have enough light in your aquarium.

Now,…

In a fish-only tank, where live plants are not present, determining whether there is enough light can be a bit more challenging. However, some signs can indicate whether your tank has enough light.

For starters…

Fish comfortable in their environment will be active and swim around the tank. If the lighting levels in the tank may be too low, they will not do much-unless you have skittish or nocturnal fish- because fish don’t see in the dark.

Tropical freshwater fish getting enough light will also have vibrant colors, while those not getting enough light may appear dull or pale.

Your fish should have enough light in the tank to feed and move around during the day, even in a dark fish room.

Reflection and shadows, especially on the sides, and darker spots near the substrate than at the top will also appear if your fish tank does not have enough light.

That said…

It’s important to note that…

…too much light or long light hour, more than the recommended period, can be harmful to fish in the tank. As such, monitoring the lighting and making adjustments as needed is essential for maintaining the health of your fish-only.

Also…

…different species of fish have varying light requirements, so choose the right amount of light based on the fish species you have in the tank.

Signs Your Fish Tank is Not Getting Enough Light

If a light is too low for your fish tank and your plants and fish are hardly getting any

• Your plants will be pale and stunted
• Your plant leaves will turn yellow instead of a lush green
• Fish and other aquatic animals will be lethargic or hide more frequently
• There will be changes in the coloration of your fish
• It will be hard to see into the tank, especially when the water is not clear
• Reflections and shadows in your fish tank

Signs Your Fish Tank is Getting Too Much Light

If your fish tank light is too strong and your plants and fish are getting more light than they need:

• Your plants will develop quickly but with thin, long, and unhealthy stalks and leaves.
• There will be increased algae growth in your fish tank
• You’ll have high water temperatures even without a heater

Well, thats all for this post.

See you in the next one.

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