Lighting for the Planted Aquarium

Lighting is one of the most important details for a planted aquarium. You have to form a balance between the intensity of the light, the duration of the light, and the availability of nutrients. Get the balance wrong, and you’ll find yourself with an algae farm on your hands.

Color Temperature

Lets start with the easiest factor of your lights, the color temperature. Color temperature is a way of expressing what wavelengths of light are most prevalent in your light source. This is generally given in degrees Kelvin, which comes from the color a black body radiates when at that temperature. A low K number will give you a warm light, this is what you are probably use to in your own home with soft white incandescent light bulbs. A warm temperature is high in the reds. On the other end of the spectrum, a high K number is high in the blues and can be described as a cool white. Opposite I know, a warm color has a low temperature and cooler has a higher temperature.

Back on topic though, the specifics here you don’t really need to worry about. What you want is a light in the 5000K to 7000K range. Often if you look at flourescent lights the “Daylight” tubes are at a temperature of 6500K which is perfect for growing plants.

Types of Lighting

There are three major types of lighting that you will see for freshwater tanks. Salt water have a few other options, but you are not likely to see them in the majority of stores. These choices are incandescent, flourescent, and LED. Right off the bat we can exclude incandescent bulbs, they are just too warm for plants and lack the blue wavelengths the plants need. Usually you only see incandescent fixtures appear on smaller tanks anyays. Sometimes 20g, but usually 10g and less.

Fluorescent Lights

Fluorescent lights are by far the most common in use. They are generally cheap, easy to find, and as long as you don’t get a dud last quite awhile. Within flourescent lights though there are 4 different packages you are likely to come across. Spiral CFLs, T5, T8, and T12.

Spiral CFLs are the screw in lights you use in your house. These work just fine often in those light fixtures on small tanks that usually come with screw in incandescent lights. You can even use them on larger tanks with those clip on light reflectors. I would not recommend using that on larger tanks though. It looks tacky, and has the risk of the light accidentally falling on or in the tank.

T5, T8, and T12 fluorescent lights are the long tubes that can come in 18″, 24″, 36″, and 48″ lengths. These are the workhorses of the aquarium world, but T12 fixtures are dying out and soon probably will never be seen again. So what’s the difference between them? Diameter. The number after the T is the number of 1/8 inches in diameter of the tube. So a T8 is 8/8 inches, or 1 inch. A T5 therefor is 5/8 of an inch in diameter  The amount of light output is roughly equivalent but as you go smaller in size they are a little bit more intense. The wattage is also a little smaller, as the smaller tubes are more energy efficient.

One thing to note however is not all T5 tubes are made equal. In the marine world people want to keep corals, corals which live in shallow waters of the ocean and get access to direct sunlight year round. They need far higher intensity than freshwater and thus typically use more tubes in parallel to up the light level. Because that all takes up space, they invented tubes that output far more light, and thus use more energy (wattage). These were called T5HO where the HO stands for High Output. For freshwater tanks you generally want to avoid using T5HO, they are just too bright. A single T5HO would often be a great choice, but unfortunately most fixtures have at least two tubes, and will not function if one of the tubes is missing.

For planted tanks, it is also necessary to replace the fluorescent tubes each year. The tubes will still be putting off light, and they’ll look the same to our eyes, but in reality they do start to lose intensity over time. This does add expense, but for many of the sizes (especially 48″) you can easily find “Daylight” tubes in any hardware or department store. For the more odd sizes like 24″ you may be forced to either order online, or pay the extreme premium for ones sold at pet stores.


These are fairly new to the market, just like how LEDs are just now starting to show up in home lighting. Because they are so new still, they are often very expensive and not the easiest to find. The real hard part though is finding the color temperature. They just don’t advertise that on a lot of the LED lights you find in stores. LEDs on their own typically only emit a single wavelength of light. That’s how you get red ones, green ones, blue ones, etc. So for fish tanks, they have to mix and match to get a spectrum of colors, which we see as white. Depending on the quality of the fixture they could have only a few wavelengths, or a more true full spectrum. The only readily available LED fixture I have seen that I know would work well for plants is the Marineland Doublebright. Aside from that you can find some more options by ordering online.

A main driving force for people wanting to choose LEDs is the power savings. It is true that LEDs use less power than any other lighting option so they will lower your operating expenses. However, since the light is only on for a part of each day, the savings is pretty small and in most cases will take a very long time before they break even in comparison to fluorescents  The real savings in them comes from not needing to replace them every year like you do with fluorescents.

A lot of LED fixtures will come with a night time light, usually blue in color. Do not use this. Fish, like humans, need periods of total darkness at night so they can sleep. That shouldn’t be a surprise to anyone. But, it is also true with plants. They need rest periods too between photosynthesis sessions.

How Much Light

Okay, now for the real question everyone has. How much light do I need? Most often people gravitate towards some WPG (watt per gallon) number. Avoid that! Those guidelines were made many years ago when lighting was far less efficient than it is today. And as I hinted at with fluorescent lights, the different technologies (T12, T8, T5, T5HO, LED) that have appeared over the years all have a different light intensity to power used ratio. Wattage is a measure of the electrical power used, not the amount of light produced. Year after year lighting gets more and more efficient. So in short, ditch any idea at all of using a magical WPG to get the perfect amount of lighting.

Why does this matter?  For two main reasons. Too little light and your plants will die, or at best just struggle along never getting very lush or large. Too much light and you’ll tip the balance between light and nutrients and algae will take over your tank. In addition, freshwater fish do not like bright lightning. They come from forests, where the streams and river banks they swim in are covered in a thick jungle canopy. Unlike marine fish who are out in direct sunlight all day, tropical freshwater fish are in dim, dark environments. In fact, many freshwater fish will show far more color under dim lighting than they will under moderate or bright lighting.

My advice will be in terms of fluorescent lights, as they are the most common out there and what most people use. As I mentioned above in LEDs, the only fixture I know of that works well in the Marineland Doublebright. If you decide to venture into LEDs the best you can do for comparison is usually to look at their Lumen ratings, or better yet Lux. Use those to compare to a standard fluorescent to get at least a ballpark comparison to light intensity.

In shallow tanks, for example a 10g or 20g long, using a single T8 tube across the length of the tank is sufficient. Go a little deeper and you can still stay with a single T8 for low lighting, but have the option of going with a dual T8 fixture to bump you up to the high end of moderate lighting. Ideal would be a single T5HO, which is roughly 1.5x the light of a regular T8, however those are very hard to find and can often be real expensive due to their rarity. On deep tanks, like my own 125g, a dual T8 is an ideal amount of light for a moderate light tank.

In all cases, some trial and error may be necessary to find the balance between the amount of light provided and the nutrients available  If you start to see algae form, cut your photoperiod down. That means the amount of time your lights are on each day. Start at 10 hours, 12 at maximum. If you find yourself with increasing amounts of algae even at 8 hours a day, consider reducing the intensity of your lighting. You can do that by either replacing the fixture with something less (for example dual to single tube) or you can also just raise the light fixture up higher above the tank.

It should be noted that any algae that forms will remain even after reducing the lighting level, but if you find the balance the total amount of algae should stop increasing. Some algae is perfectly fine and will always be present in an aquarium, but out of control algae can start to take over your plants and can even kill them.

Lighting is often the hardest decision with getting started on a planted aquarium, so by all means ask questions!

<– What are my substrate choices? What is best? | Do I need fertilizer? What kind? –>

2 thoughts on “Lighting for the Planted Aquarium

  1. Hi. I posted a couple of questions on filter you reviewed. Your heater review was very helpful, too. Now, I wondered about lights…..I have a new 90, lightly planted, freshwater Rainbow fish tank. I can’t afford fancy LED lights, and the reviews I have been reading everywhere on name brand lights are dismal and scary!! Is there a good, dependable, t5 system that will give light for a low tech aquarium, show fish nicely, and not encourage algae? That the ballasts don’t fail in 3 months, that bulbs actually work, that don’t short out or overheat, etc, etc, etc —- Or is that wishful thinking?

    Any comment appreciated.

    • You will not likely find a T5 system that will be low enough light, especially T5HO. T8 lights are more reasonable. My 125g aquarium uses a dual T8 setup which is sufficient.

      If you can find a single bulb T5 fixture that will be okay, but most are 2 or 4 bulb fixtures and have ballasts that do not allow you to load a single bulb.

      I’ve never had a ballast fail on my aquariums, so I wouldn’t worry about that. Bulbs are also independent of the fixture. They’re a standard size so you’re not locked into something custom. I always go for a fixture that has a bulb length and size (I only use T8) that I can find in a place like Home Depot or Lowes. The daylight bulbs (~5500K) work perfectly for plants and are a fraction of the cost of what you’ll find in pet stores.

      LED lighting *can* be fine, but tends to be expensive and too dim. I’m an electrical engineer by day, so I built my own for a 20 gallon aquarium.

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