Choosing the best light spectrum for planted tanks

Lets start by reading spectrum charts

An example of a light spectrum diagram for a T5 lamp is shown on the left below. The amount of light produced for a particular segment of the spectrum is the area under the curve. This bulb has a balance of red/green/blue which gives the bulb a relatively neutral white appearance overall. The spikes come from special phosphors used in the manufacture of fluorescent lamps. Different bulbs will have different peaks in their spectral profiles. Visually, it can be difficult to extrapolate the visual colour of bulbs from reading spectral profile charts unless you have a lot of experience. It is always a good idea to inspect aquarium lamps in person.

The spectrum also changes with depth. Water absorbs red light more readily than blue, which has a higher frequency/energy. Approximately 30% of the red light is lost at a depth of 2 feet. The spectral graphs below show how the spectral curves shift in water. (The Y axis of the spectrum chart is relative.) Manufacturers rarely go to such lengths to show this data.

Most standard aquarium lights are deficient

A 6500K plain white LED diode (left) versus a 2700K warm white LED diode (right). These are commonly used for household lighting and often make up the bulk of cheap aquarium LED bulbs. Plain white 6500K diodes lack an adequate red spectrum, so using LED fixtures with plain white LEDs often results in washed out visual colours and poorer pigmentation in red plants, although plants may still grow. Warm white LEDs often work better for growing plants, but they give a yellowish appearance to the aquarium due to a lack of sufficient blue spectrum.

Many commercial T5 LED fixtures come with a spectrum that I find lacks red. This is particularly true of LED fixtures that use pure white diodes. They will have an overall spectrum profile similar to the 6500K pure white LED diode above, which may grow plants well, but will give poor visual saturation and contrast, and will also be worse at growing red plants than fixtures with a higher red spectrum in their profile.

A better spectrum profile

Why red and blue parts of the spectrum are important in planted aquarium lighting.

Better visual presentation on coloured plants. Most white lights (bulbs/LEDs) lack red/blue by default. This makes aquariums look washed out. Coloured plants pop much more when there is more red/blue light. However, there must also be enough green/orange/yellow spectrum to give a balanced visual output.

Better coloured pigmentation in certain plants. While all wavelengths of light between 400nm and 700nm contribute to photosynthesis, stronger red/blue stimulates pigmentation in certain plants. This means that many red plants will become redder when exposed to a strong red/blue spectrum. Many people think that good pigmentation in coloured plants is achieved by nutrient levels alone - when in fact lighting plays a very important role.

Anecdotal evidence also suggests that stronger red/blue light produces plants in significantly better growth forms - plants grow more compact with fuller leaves. 

What to look for in planted tank light spectrum

For T5 lighting

At the minimum, we recommend 50% of the tubes in a T5 array to be changed to pink/red gro-tubes. (meaning 2/4 of a 4 tube T5 array). Our own farm tank's T5 tube selection is shown here (no white tubes !) :

This tank was created using the T5 bulb selection above, which gives fairly strong colour saturation in the reds and blues, while still allowing different shades of pinks and oranges to be well represented. If only pink/blue bulbs were used, the tank could appear visually over-saturated with red - and mid-tones such as orange and pink would appear more reddish than their respective colours.

For LED lighting

RGB (Red, Green, Blue) LED fixtures that use a mix of red, blue and green diodes (and fewer pure white LEDs) have high peaks in the red, blue and green wavelengths. These types of luminaires perform much better than LED luminaires using pure white LEDs. The few commercial LEDs that do this very well include the Chihiros, ADA solar RGB, Netlea and WEEK Aqua series. These all have large RGB spectral peaks with their red, green, blue diodes, but with large spectral gaps in between. This spectral profile gives very high saturation for reds, blues and greens, which makes red plants really pop in the tank. This is much more obvious in real life than you can see in videos/photos.

Spectrum profile for the ADA solar RGB LED below:

This tank above and below is grown using Week aqua a430 series, which gives a nice neutral white back ground tone yet high color saturation for red plants.

A comparison of ADA solar RGB and some other lighting fixtures can be seen in this video:

A comparison of Chihiros vivid (similar spectrum to ADA solar RGB) and other chihiros fixtures:

Other tank examples

The T5 selection favored by Tom barr shows liberal use of red/blue tubes.

The T5 selection by Joe Harvey forgoes white tubes almost entire, and focus on a mix of pink/purple tubes with a yellow tube for balance. Surprisingly, due to human eye sensitivity to green, it takes only a little to counter balance strong red/blue tones.

The Twinstar series of LED are popular; the S series is what you want. The cheaper version of this light that is similar is the Chihiros WRGB. Twinstar S probably has better build quality but comes at higher price.

When to use more green?

Contrary to the examples above, older ADA lighting systems have developed light spectrums with high amounts of green rather than red/blue (i.e. NAG Green bulb for their metal halide fixtures). These will wash out red plant colours but will work to make aquascapes with only green pop.

If you are attracted to intense shades of green, choosing fixtures with a high green spectrum will work to give the tank a green glow.

Spectrum correlation to algae?

There is a persistent myth that more blue light means more algae. Having seen thousands of aquarium examples, we don't see this correlation at all. This is a consistent observation by other expert growers such as Tom Barr. Algae is always closely related to overall plant health and tank cleanliness, organic waste levels. Higher light levels will trigger algae more quickly in tanks with trigger factors, but spectrum (as far as hobby tanks are concerned) is not a factor.

Here are link to learn more about:

1. Algae control

2. T5 tubes vs LED