'Watt' to Look at When Buying Aquarium Grow Lights

Posted by Augusta Hosmer on

Read Time: 10 mins

For most folks, lighting is easily the most intimidating part about setting up a new freshwater planted tank. If you're looking to buy and are frustrated trying to figure out what the hell watts, PAR, Kelvin, and CCT all mean for your aquarium plants, we've got it all laid out for you, step by step. 

1. First Thing to Look at: Your Budget and Your Goals

Grow lights can run anywhere from 10 bucks to hundreds or more. There's no sense planning out an epic planted tank only to realize you can't afford to light it (for tips and tricks to cut down on costs for your aquarium, check out our blog on "14 Ways to Make Your Aquarium Cheaper").

You should also figure out just what you plan to do with your tank. If you hope to set up a lush aquascape you could enter into competitions, you're going to be more concerned about light than someone who just wants to keep a few easy plants alive or is simply looking for filter plants.

2. Second Thing to Look at: Your Tank

Fish tanks come in all shapes and sizes, and so do grow lights. You should measure your tank dimensions before you buy lights to figure out the length of the light you need. Some lights' lengths are adjustable. If you have a really long tank and want plants all the way across, you may need to look at getting multiple lights, or centering the light and putting lower-light plant species towards the edges of the tank. 

4 Aquarium Lights of Varying Length

Above: Four LED aquarium lights of varying lengths

Many hobbyists stop at measuring their tank's length since that's how grow light sizes are advertised, but you should also look at how tall your tank is. The deeper your water, the more intensity is needed for your plants on the bottom of the tank to get the light they need.

You may have a kickass grow light for a shallow tank, but if you're trying to grow a carpet in a vertical tank and haven't accounted for that extra depth, you're asking for problems. Water absorbs red and yellow light more easily than green and blue, so if you have a deep tank, you should make sure you're getting enough of the warm side of the spectrum in particular. 

A graph showing how different colors of light penetrate deep water. Blue, purple, and green wavelengths reached further, while red light was absorbed more quickly and only reached the shallows.

This graph (from Wikipedia Commons) shows how different light wavelengths penetrate water. As you can see, red light is unable to travel as deeply as blue, purple, and green light because water more easily absorbs it. 

You should also look at how the rest of your tank is situated, if you have an otherwise complete setup already. If your tank is on a shelf or entertainment system, that could limit the amount of vertical height you have to place and adjust your lights. If your tank doesn't have a lid or a means to place a light on top of it, that means you need a way to safely clip or hang your light above it.

You should also make sure your lights' power cords/plugs are managed in such a way they're unlikely to get splashed, snagged, frayed, or overloaded, because aquariums are one of the leading causes of house fires. 

Lights also generate heat (some kinds more than others), so you need to make sure wherever you put your lights can handle that and isn't flammable. 

3. Third Thing to Look at: Your Plants

If you haven't bought aquarium plants yet, then great! You have a lot more freedom when it comes to choosing your lighting setup and then matching plants to it later. But if you already have some plants growing under a cheap light (or no light. It's okay; I, too, like to live dangerously) and you're looking to upgrade to keep them alive, you need to figure out the lighting needs for any current or future species. 

If you have or want a lot of floating plants in addition to plants in your substrate (especially species like duckweed that form mats on the water surface) you should factor in the fact your light has to make it through them to reach the bottom. You may need to buy a stronger light or thin the plants casting shade. If that plant is duckweed, may the Force be with you. 

If you want to grow some red plants and keep that gorgeous, deep color, you'll need more light too.

  • Plants you can grow with cheap/low lights: Java Ferns (Microsorum); Anubias; Banana lilies (Nymphoides aquatica); Bacopa; Anacharis; Water wisteria (Hygrophila difformis); Floating species (red root floater; water hyacinth; water lettuce; hornwort, etc.); Mosses (Java moss, Flame moss, etc.); Cryptocoryne
Limnophila aromaticoide

4. Fourth Thing to Look at: Your Animals

You can stress a little less about light shopping if you're not keeping live plants and just want some fish. Fish need a light and a dark period to establish circadian rhythm (yes, they do sleep in their own unique way), but this can be accomplished even with cheap lighting. 

That said, there are some aquatic animals (axolotls, in particular) that are sensitive to light, so you should keep that in mind and research your stock.

5. Fifth Thing to Look at: Types of Grow Lights and the Pros and Cons of Each

These are the four main types of grow lights on the market, and they all have advantages and disadvantages. Here are those laid out to help you make your decision: 


How they work: Electricity passes through gas in a bulb (often mercury); the gas ionizes and emits light 

Pros: Longer life/more energy efficient than incandescents; lower heat than incandescent or metal halide bulbs; lots of different options and types available to suit specific tank needs

Cons: Shorter life/less energy efficient than LED's


How they work: These emit light via heated metal. The metal filament in an incandescent bulb (usually made of tungsten) heats up when electricity passes through it, and this releases light. 

Pros: Low cost; readily available 

Cons: Shorter lifespans; least efficient; fragile

Light-Emitting Diodes (LED's)

How they work: Small bulbs are fitted into an electrical circuit, and electricity is passed through a semiconductor to emit light

Pros: Longest lifespan of the types on this list; the most energy-efficient; do not emit as much heat; durable

Cons: Usually more expensive

Metal Halides

How they work: Work similarly to fluorescent bulbs, but with a different gas composition. Electricity passes through a mix of gas, and this reaction emits light. 

Pros: More efficient than incandescent bulbs with better light quality and higher light intensity; can be useful for deeper tanks

Cons: Longer warm up times; maintenance costs; produce a lot of heat; more expensive than fluorescents

    6. Sixth Thing to Look at: The Grow Lights' Labels and Measurements

    This is the part that causes the most hangups, but it doesn't have to be a headache. Your goal is to imitate sunlight without burning something or dropping so much money you need to sell your liver. That's it.

    Before we get into definitions, you should understand how light and color work. Light has 

    Now that's out of the way, here's what some of those terms and abbreviations on light product labels mean and what to do with them (in alphabetical order):

    • CIE - the abbreviation for the International Commission on Illumination (or Commission Internationale de L’Eclairage in French, which is where CIE comes from). They provide scientific standards for lighting measurements and publish a lot of those color spectrum charts you see on lighting pages. They also have an enormous library of scientific publications and guidelines on the physics of light and color. I got a lot of the info for this post from them (here's a link to the CIE's searchable list of definitions if you want more: E-ILV).
    • Color Rendering Index (CRI) - this measures ability of light to reveal colors accurately / how similar artificial light is to sunlight on a scale of 0-100. The closer to 100 the CRI is, the more similar it is to sunlight. Should be used alongside CCT.
    • Correlated Color Temperature (CCT) - Measured in Kelvin (K), CCT can tell you the color/warmth of light being emitted. A higher K value means light on the blue side (cool light), while a lower K value means light on the redder/yellower side (warm light). Sunlight usually falls between 1850K (at sunrise/sunset) - 6500K depending on conditions - even over 10000K sometimes. CCT is a useful metric, but there is a ton of variation in this. So don't use this measure exclusively to pick your lights.
    • "Full Spectrum"- I honestly just completely ignore this when I'm looking at light fixture labels, because it doesn't tell you anything useful. It's to the point now if I see this, it's almost a red flag. Oh, okay, sure, your light is white. Congratulations. That's all "full spectrum" means, since any white light includes the full color spectrum. How that spectrum is composed in a particular fixture is more important (more on that below). Plus, you can make plants grow like crazy without the full spectrum. Look at red, pink, and blue grow lights. 
    • Kelvin (K) - unit for light temperature; what CCT is measured with. Higher K = cooler light (see CCT). It's important to note the Kelvin used to talk about light is very different than the temp in Kelvin of a physical object like you'd see used in biology, chemistry, or other subjects in physics. The temperature of the bulb is not actually approaching 6500K or 12000K or whatever your CCT is. Just to give you an idea, 6500K is 11240.33 degrees Fahrenheit (~6226 degrees Celsius). The surface temp of our sun is ~10,000 degrees Fahrenheit, which could be slight overkill for an aquarium heater, but I'm not going to tell someone potentially holding the sun in a glass ball how to take care of their fish. 
    • Lumens (lm) - unit for how bright a light is. The unit used for brightness used to be exclusively watts (W), but that was with older incandescent bulbs where more watts always = more brightness. But now, with the birth of LED's, some bulbs can produce more light with fewer watts. This is why many LED's are touted as being more energy-efficient 
    • Lux - describes intensity and how well a light illuminates an area. Equals 1 lumen per square meter. Higher lux will cover more ground in your tank. 
    • Photosynthetically Active Radiation (PAR) - describes the range on the light/color spectrum that photosynthetic organisms (plants, algae, phytoplankton, etc.) can use. PAR spans 400-700nm (see image below)
    The visible light spectrum
    • RGB - Red, green, and blue lights; usually a type of LED. These lights have more red, green, and blue value on the color spectrum profiles (you can see how they peak at red, blue, green). Since plants use these light colors more readily, a fixture with more RGB bulbs than simple white light bulbs will help plants perform better. 
    • Spectrum/Spectral Composition - if you're seeing pictures of a rainbow on a graph for a grow light, it's showing the amount of certain available light colors it provides to your plants. Plants need red and blue light in particular, but utilize all visual light between 400-700nm on the color spectrum (see PAR). I look for lights with a spectral composition with peaks in red, blue, and green (see RBG). Simply saying a light provides full spectrum isn't enough - it's HOW that spectrum is composed. A regular old lamp bulb provides a full spectrum, but it won't give you the peaks in red and blue your plants like. If possible, try to see the lights in person or find a good pic/video of them already set up with a tank, because how they change the appearance of your tank varies a lot. 
    • Watts (W) - unit for how much power is consumed by a light source. A lot of people confuse watts with lumens, but they measures energy usage, not brightness. A 60W bulb will use more energy than a 40W bulb, for example, but may or may not be brighter.

    So, to sum up what you want to see on an aquarium grow light product label if you want a top of the line light, you're looking for something that:

    • Provides more lumens with fewer watts (energy efficiency; look for lumens per watt measurements)
    • Has a long lifespan (should be indicated on the label somewhere)
    • Has a CRI approaching 100 (shooting for above 85 is a good target) and generally in the 6500K CCT range if you want a natural sunlight look, but you can grow plants all over those scales. So don't eliminate a light solely because it's not 6500K. Maybe you'll like the look of a cooler light closer to 12000K better - you can still make it work if your other light metrics are solid. 
    • Has RGB lights or peaks in red/blue on its spectrum chart - preferably with graphs/data for varying depths since as we discussed above, water absorbs red light more easily than blue. 

    You don't want to look at any of these metrics in isolation. Finding a grow light that ticks most of those boxes while staying in your budget is the way to go, and you may have to experiment to figure out exactly what works for your particular setup. 

    7. Seventh Thing to Look at: Accessories / Bonus Features for Grow Lights

    Though the lights themselves are the most important part, there are also some separate accessories or even built-in features that can make working with them easier or more fun.

    The accessory we recommend 100% of the time is a light timer. These are cheap, readily available, and easy to use. Some have multiple outlets so you can use one timer for more than one light. A light timer will allow you to save on energy costs and ensure the exact amount of time your plants and fish are exposed to light each day (AKA light duration or photoperiod). It also cuts down on one more thing you need to remember to maintain your tank. These can be found in most hardware stores. I got a two-outlet timer for $9.99.

    Some lights come equipped with a controller with multiple brightness settings. 

    It's also helpful if lights have an adjustable stand or movement mechanism so you can increase or decrease distance to your plants as they grow or you cut them back, if needed. These often come with a plastic bracket to secure the light where you need it on the tank.

    However, some companies also make submersible LED lights. It's important to note most aquarium lights are water resistant or waterproof, but that doesn't mean you can safely submerge them unless they're specifically designed for that. 

    A submersible LED Aquarium Light

    A submersible LED Aquarium Light

    Some lights have sunrise/sunset/moonlight/ strobe features that gradually turn your lights on and off and are designed to simulate natural light patterns. Is it 100% necessary? No. Is it cool, and most likely slightly less stress on your fish for a few moments when you turn lights on every day? Yes. Am I easily taken in with unnecessarily fancy lighting features and tempted to throw an occasional EDM rave in my planted tanks without fish? Also yes. If you think it's neat and want to get it, go for it, but your fish/plants won't suffer without it.

    Looking for more tips and tricks on aquarium lighting? Head over to the Dustin's Fishtanks YouTube page for some helpful videos: 

    Top 5 Lighting Tips For Your Aquarium

    Planted Tank 101: LIGHTS - The Best Aquarium Lighting

    How to Use Sunlight in Your Aquarium

    Aquarium Lighting Tips: Are You Afraid of the Dark?


    Tank on!

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