Planted Tank Lighting 101: Understanding the basics of planted aquarium lights

The tank below uses a custom made BML LED bar on this tank. You can see the mix of red, blue and warm white LEDs mixed with neutral white LEDs being used. The fixture provides a large height clearance above the tank, allowing pruning work to be carried out without the need to move the lights - to achieve this the LED bar uses a narrow 45 degree lens. The light bar is 34 inches from the tank floor and yet we can grow demanding carpet plants in the scape. With the right knowledge we can customise our lights to suit our aquascaping needs. In this particular setup, it is not that the fixture is super powerful (although it is very efficient at only 75 watts), it is that it uses a narrow angled lens (25 degrees) to focus the light produced on the bottom of the tank. The light will not work if mounted lower as it will not cover the entire tank.

The right aquarium light affects everything: it can enhance the colouration of aquarium plants, determine growth rates and affect algae control. We need to determine the appropriate wattage and colour spectrum, and cut through the scientific jargon that is often thrown around.

No other subject is as full of misinformation and marketing gimmicks as aquarium lighting, for example the idea that 6500K is an "ideal" spectrum - my lamp above is rated at 3600K, which is very different from a simple 6500K lamp, and we have grown tanks with 12000K lamps as well. The K rating is not an indicator of whether or not a light is suitable for growing plants. It simply measures the visual hue of the light. However, lamp manufacturers will claim that 6500K is similar to daylight and give the light labels such as 'full spectrum'. Neither of these are really important when choosing a lamp.

A mix of tanks grown under different K-rating lights.

"Full spectrum" is also a marketing term that has nothing to do with how well a light grows plants. Any white light can be described as full spectrum as all white lights contain RGB (red, green, blue) wavelengths of light by default. Furthermore, you can grow plants just as well without full spectrum light. For example, the International Space Station uses only red & blue LED diodes to grow plants.

Even many biology textbooks have yet to be updated to reflect new data in this area, such as the fact that plants use a significant amount of green light for photosynthesis.

So what really matters? 3 important factors to evaluate a light fixture

Factor 1: Strength - measured in umols of PAR

PAR (Photosynthetically Active Radiation) is the most accurate measure of the 'strength' of light relevant to plant growth, as it directly measures the amount of light available for plant photosynthesis. PAR figures are provided by most reputable luminaire manufacturers. Click here for information on how to read manufacturers' PAR charts. If PAR values are not provided by the lamp manufacturer, you can often find values provided by hobbyists who have PAR meters.

What if no PAR tables or hobbyist data are available? The other way to judge whether a light will grow the plants you want is to see tanks with that particular light. If other hobbyists using the same lamp can grow the same plants you want to grow, chances are it'll work.

Why Lumens and watts are poor indicators for light strength

Lumens is what some outdated websites use, but it is a poor measurement because it measures the brightness of light in relation to the sensitivity of the human eye to the electromagnetic spectrum of light. This means that lights that are high in green wavelengths have a higher lumen rating because the human eye is more sensitive to green than red or blue. However, plants use red and blue light efficiently for photosynthesis, so red/blue heavy lighting may have low lumen values but can be really great for growing plants. Similarly, watts per gallon was the old rule when fluorescent lighting for aquariums was the norm. However, watts measure how much electricity a fixture consumes - not how much it produces. An inefficient lamp may have a very high wattage, but much of the energy is wasted as inefficient heat rather than light energy. The tank at the top of the page uses a 75 watt LED fixture on a 65 gallon tank (1.2 watts/gallon) - despite being mounted 16 inches from the edge of the tank, it still produces enough light to grow colourful plants well because of the efficiency and lens of the LEDs used.

The farm tank below uses T5 tubes; (39w x 8 tubes = 312watts). Over a 46 gallon tank this works out at 6.8 watts per gallon. This is a huge difference to the LED system used in the top tank.

How much PAR should a tank have?

The light should produce sufficient PAR at the substrate depth of your aquarium. The broad guidelines as commonly used by the aquatic plant community are as follows:

Using a PAR meter will show that this tank has 128 umols of PAR at this point on the substrate. Areas that are shaded will have lower readings. Lighting manufacturers often publish PAR tables to show how much light a fixture will produce at a given depth of water. Alternatively, many hobbyists own PAR meters and take their own readings.

How much PAR do you get by squeezing 8 x 39w T5 tubes into a 90x45x45cm tank? Around 200+ based on PAR meter readings. Extrapolating from this; most 90x45x45cm tanks will grow well with half the number of T5 tubes we use, 4 x 39w should give around 100+ umols of PAR.

This article explains how to read PAR tables by different manufacturers

A 4 tube T5 unit over a 2 feet tank is cheap and still produces a lot of light.

Downside of strong lighting?

The main disadvantage of too much light is that it increases the chance of algae growth. This sounds trivial, but managing algae is probably the biggest challenge for beginners in this hobby, so it is wise to use lower lighting if you only have shade plants.

Using very strong lighting without good tank fundamentals will cause spectacular algae blooms. However, if a tank is run on poor fundamentals, using low lighting doesn't guarantee an algae-free tank - algae will just grow more slowly. The most important thing to remember when dealing with algae is to remove the potential triggers. Even tanks with 100++ umols of PAR on the substrate can be kept completely algae free if the substrate is free of organic waste and the plant mass is healthy. See the algae page for more details.

However, stronger lighting combined with good fertiliser/CO2 will bring out richer colours in coloured plants.

Factor 2: Colour spectrum 

The goal is a balanced spectrum with emphasis on the reds and blues. The exact way to do this is to look at the spectrum graph itself - if available. The K value is only a loose proxy.

Which spectrum works best? Heavier reds/blues improve contrast of fish and plants

While aquarium plants use all colours of the spectrum for photosynthesis, we find it beneficial to have stronger reds/blue in a fixture as this provides better colour contrast for the colours commonly seen in aquariums. A stronger red/blue spectrum also stimulates colouration in red/coloured plants. We will never use an LED fixture with plain white LEDs because of the lack of spectrum of plain diodes.

Ultimately, the main deciding factor is that the spectrum has to look good to the aquarium owner. Aquariums are a visual art and aesthetic colour rendering is essential. One basic way to choose a lamp is to copy the lighting choices of tanks you like. You can do this by looking at pictures of tanks on the internet, but due to the problems of photography this may not be as accurate as seeing the light over a tank in person.

The K rating of a light gives the approximate colour hue of the light but doesn't tell whether the light has a good spectrum distribution or not.

Click here to read what K rating measures.

Reading Spectrum Charts

Most reputable light manufacturers will publish spectral charts for their lights. Below is the spectrum chart of a BML LED light unit and my tank under the light. The amount of each colour of light produced is equal to the area under the curve. This particular light unit has large blue and red peaks and produces little yellow and cyan light. This spectral profile highlights the reds and blues in the tank.

What is important is the relative area/size of the peaks. To appear neutral white, a lamp will have peaks in blue, green and red. A fixture that is all blue and red, with no green, will appear pink/purple and cast a reddish hue over the aquarium. In this way we can roughly estimate the overall colour rendering of the light by reading the spectrum chart.

Does a higher CRI rating on the luminaire make a difference? In short, it does not. For most aquarists, colour accuracy is not as important as colour saturation. The CRI measures the colour accuracy of lighting units, whereas hobbyists overwhelmingly prefer lighting that has high colour saturation and contrast. Take the aquarium at the top of this page again; it's CRI is only 78, but it gives good colour saturation and contrast for viewing.

This article explains how the CRI is actually calculated, and why it does not really matter.

Higher levels of red/blue give better colour contrast and pigmentation in plants

A higher percentage of red/blue in the lights will produce plants with better pigmentation (stronger colours) and also give better visual contrast saturation. This tank is grown using the T5 arrangement below with a mix of red/blue and orange bulbs.

 Read more here on what light spectrum planted aquariums should aim for 

Factor 3: SPREAD

It is important that the spread of the fixture matches the dimensions of the tank. We have found that dual LED fixtures or a T5 array work well for complex hardscapes with shaded areas, as having two or more light sources tends to reach most areas of the planted tank. A low spread fixture (e.g. single bulb) will have very high PAR in certain areas (often the centre of the fixture) while the edges will be shaded.

Point sources have circular areas of coverage and are best suited to square planted aquariums or tanks that can be divided into squares with minimal overlap. Aquascaping rocks and other tall hardscape can easily block point source light from reaching the plants. Using 2 fixtures, front/back, or going for lighting kits that come in an array may work better.

A wide-angle LED lightbar has a dispersion pattern where higher parts of the hardscape can block light from reaching plants behind the hardscape. Object simulated by the blue block in the diagram.

A more distributed light source, such as a wider LED array or a T5 array, will not have a similar problem.

Here are links to further reading:

1. Algae control

2. How to read PAR tables

3. What light spectrum do plants use for photosynthesis

4. What does the K in 6500K actually measure

5. What light spectrum work best for planted aquarium light fixtures