The tank below uses a custom build 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 gives a long height clearance above the tank, which enables pruning work without the need to move the lights – to achieve this the LED bar uses a narrow 45 degrees lens. The light bar is 34 inches away from the tank floor, and yet we can grow demanding carpeting plants in the scape. With the right knowledge we can customize our fixtures to suit our aquascaping needs. In this particular setup, it is not that the light is super high powered (though it is very efficient at only 75 watts), it is that it uses narrow angled lens (25 degrees) to focus the light produced onto the floor area of the tank. The light will not work mounted lower as it will then not cover the whole tank.

The right aquarium light affects everything: it can increase aquarium plant coloration, determine growth rates, and impact algae control. We need to determine suitable strength as well as ​colour spectrum, and cut through the science jargon that tends to be thrown around liberally here.

No other topic is filled with as much misinformation and marketing gimmickry as aquarium lighting, for example the idea that 6500K is an “ideal” spectrum – my light above is rated at 3600K, far different from a plain 6500K light and we have grown tanks with 12000K lights as well. K rating is no indicator of whether a light is suitable to grow plants or not. It simply measures the visual color hue of the light. However, light manufacturers will claim that 6500k is like daylight and assign tags such as ‘full spectrum’ to the light. Neither of those points are actually important in choosing a light.

A mix of tanks grown using different K rating lights.

“Full spectrum” is similarly a marketing term that has no bearing on how well a light grows plants. Any white light can be labeled full spectrum as all white light contain RGB (red, green, blue) wavelengths of light by default. More over, one can also grow plants just as easily without full spectrum light. The international space station uses just red & blue LED diodes to grow plants for example.

Even many biology textbooks have also yet been updated to reflect new data in this area; for example, that plants are found to use a significant amount of green light for photosynthesis.

So what actually matters? 3 important factors to grade a light unit

Factor 1: Strength – measured in umols of PAR

PAR (Photosynthetically Active Radiation) is the most accurate gauge of a light’s relevant ‘strength’ for plant growth as ​it directly measures the amount of light available for plant photosynthesis. PAR numbers are given by most esteemed light manufacturers. Go here on how to read manufacturer’s PAR tables. If PAR readings are not given by the light manufacturer, you can often find readings taken by hobbyists who own PAR meters.

What if no PAR tables or hobbyist data is available? The other way to gauge whether a light will grow the plants you want is to see tanks that are running that particular light. If other hobbyists using the same light 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, however, it is a poor measurement as it measures light brightness with respect to human eye sensitives to the electromagnetic light spectrum. This means that lights that are heavy in green wavelengths have a higher lumen value as human eyes are more sensitive to green than red or blue. Plants however, 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 used when fluorescent lighting for aquariums was the norm. However, watts measure how much electricity a light unit consumes – not how much it outputs. An inefficient light may have very high wattage, but channel a lot of the energy in inefficient heat rather than light energy. The tank at the top of the page uses a 75 watt LED light on a 65 gallon tank (1.2 watts/gallon) – despite being mounted 16 inches from the tank’s rim, it still produces enough light to grow colored plants well because of the efficiency and lens of the LEDs use.

The farm tank below uses T5 tubes; (39w x 8 tubes = 312watts). Over a 46 gallon tank, it comes in at 6.8 watts per gallon. This is a huge difference from 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 tank. The broad guidelines as commonly used by the aquatic plant community is as follows :

Using a PAR meter shows that this tank has 128 umols of PAR at that point on the substrate. Areas that are shaded will measure lower values. Light manufacturers often publish PAR tables to show how much light is produced by a light unit at a certain water depth. Alternatively, many hobbyists own PAR meters and take their own readings.

How much PAR will you get if you squeeze 8 X 39w T5 tubes onto a 90x45x45 cm tank ? Around 200+ going by PAR meter measurements. Extrapolating from this; most 90x45x45cm tanks will grow well with half the number of T5 tubes that we are using, 4 x 39w will be expected to 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 downside of having too much light is increased chances of triggering algae growth. This sounds trivial but managing algae is probably the biggest challenge for beginners in this hobby, so it is smart to use weaker lighting if you keep only shade plants.

Using very strong lighting without good tank fundamentals trigger spectacular algae blooms. That being said, if a tank is run on poor fundamentals, using low lighting doesn’t guarantee an algae free tank – algae just grows more slowly. The important thing to remember when facing algae is removing the potential triggers. Even tanks at 100+++ Umols of PAR at the substrate can be kept perfectly algae free if it is clean of organic waste and plant mass is healthy. More details on the algae page.

That being said, stronger lighting, coupled with good fertilisation/CO2 brings out richer colours in coloured plants.

Factor 2: Colour spectrum

The goal is a balanced spectrum with emphasis on the red and blues. The accurate way to is to look at the spectrum graph itself – if available. The K rating 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 advantageous to have stronger reds/blue in a fixture as it gives better colour contrast for the colours commonly seen in aquariums. Having stronger red/blue spectrum also stimulates coloration in red/coloured plants. We will never use an LED fixture with just plain white LEDs due to the lacking spectrum of plain diodes.

Ultimately the main deciding factor is that the spectrum has to look visually good to the tank owner. Aquariums are visual art, and aesthetic color rendering is essential. A basic way to choose a light is to copy the light choices of tanks that you like. You can do this by browsing tank pictures on the internet, but due to the issues 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.

Read more here on what K rating measures.

Reading Spectrum Charts

Most serious light manufacturers will publish spectrum charts for their light units. Below shows the spectrum chart of a BML LED light unit and my tank under the light. The amount of each color light being produced is equivalent to the area under the curve. This particularly light unit has large spikes in blue and red, and produce little yellow and cyan light. This spectrum profile highlight reds and blues in the tank.

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

Does a higher CRI rating on the light matter? In short, it does not. For most aquarium hobbyists, color accuracy is not as important as color saturation. CRI measures color accuracy of light units whereas hobbyist over overwhelmingly prefer lights that have high color saturation and contrast. Again referring to the tank at the top of this page; it’s CRI rating is only 78, however, it gives good color saturation and contrast for viewing.

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

Higher amount of red/blue gives better color contrast and better pigmentation in plants

Higher percentage of Red/Blue in lights grows plants with better pigmentation (stronger colors) and also gives better visual contrast & saturation. This tank is grow 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

Here, the important thing is that the fixture spread must match tank dimensions. We have found that dual LED fixtures or a T5 array serves complex hardscapes that have shaded areas, as having two or more light sources will tend to reach most areas of the planted tank. A setup with poor spread (e.g. single bulb) will have very high PAR values in certain areas (often the centre of the light) while the edges are shadowed.

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

A wide angled LED light bar has a dispersion pattern where higher pieces of hard scape can block light from reaching plants behind the hardscape. Object simulated by blue block in the diagram.

A more distributed light source; such as a wider LED array or T5 array will not face a similar issue.