Reef tanks are incredibly complicated things. The animals within our tanks are that complication.

How many of us really know how to properly care for them correctly? – By large, most us have the care requirements of our fish down to a ‘Tee’, feeding correct food types, at correct intervals and maintaining fish health via correct quarantine procedures.

But how many of us are able to care for corals and give them what they really need.

Lighting and flow are 2 hugely important factors to the corals within our aquariums and many hobbyists assume that as long as they keep the major and minor trace elements in check (i.e alkalinity, magnesium, calcium) and their unwanted nutrients low (i.e nitrates and phosphates) that they can pretty much do most things with flow and light and achieve success.

In some instances this may be true, but, probably more by luck than judgement!

Lighting is arguably one of the most important aspects of coral care and so today were going to take a look in some detail about what lighting our reef tanks really need and how in tune we are with those requirements.

In this video we will be taking a look at the different types of lighting available, LED, T5, Metal Halide etc, but first we need to look at what these lights are actually producing, and how we understand those details.

Firstly PUR and PAR are 2 measurements often thrown around when we talk about lighting. But what do they really mean?

Let’s start with PAR. Photosynthetically Active Radiation. Keeping this in its simplest form, PAR is essentially the number of light particles (called photons) that fall within a square meter over the course of 1 second. PAR does not differentiate between the wavelengths of light coral use. It essentially measures the ‘brightness’ of the light without taking into account the wavelengths used. By wavelengths we mean the spectrum – i.e the colouration of the light blue, white, red etc.

PAR is important because it shows us (when measured using a PAR light meter like a Seneye or Apogee) how much light our corals (and of course lots of other reef tank inhabitants) have at their disposal for photosynthesis. PAR is measured light on the visual light spectrum and the visual spectrum (i.e what our naked eye can see) is somewhere between Infrared and Ultraviolet. IR & UV are of course waves that cannot be seen as they about a billionth of a millimetre wide and so they are not on the ‘visual spectrum’, simply because we can’t see them!

Too much PAR is bad, and too little PAR, well that’s also bad! – It’s like if a human eats too many calories. That human would become overweight and unwell. If a human also ate not enough calories they would become malnourished and also become ill. Similarly in corals when there is too much light, photo synthesis stops and the coral becomes ill this is called phot inhibition. When there is too little light, similarly to humans, the coral will become ill.

Corals are no different to humans in that they require food to survive. Light to a coral is food. They require a set amount of light to stay healthy. Usually 200-300 PAR for MOST SPS corals and between 100-200 PAR for most LPS & Soft corals. Typically SPS corals are found in shallow water and therefore closer to the sun, hence their desire for high levels of light. Corals like Zoas and mushrooms are usually found much deeper waters hence the need for darker conditions (i.e less PAR) and usually a slightly different spectrum .

Spectrum, another new word! So what is Spectrum?

White sunlight is made up of all the colours of the rainbow. We call these different colours wavelengths and they are measured in nm (nano meters) the visible spectrum goes from 400nm to 700nm. A slight variation in the red or blue changes what we call the kelvin value of sunlight. Metal Halide lamps emit all wavelengths like the sun however with LED we can control the spectrum and even create gaps where there are no wavelengths. So coming back to our human food analogy, if PAR is like Calories, then spectrum is the food groups. Having enough calories in your diet but all of it coming from donuts will not make you grow well either.

So why also PUR not just PAR and Spectrum.

Now PUR is also an incredibly important measurement. PUR stands for Photosynthetically Useable Radiation.

PUR in a nutshell is the usable portion of PAR . Ill break that down a bit more. If we imagine an overall PAR of say 300. That light is made up of a variety of different spectr um. Probably from the low 400nm range, and then up as high as the 700nm range. All corals have a different photosynthetic need and therefore will have a different PUR range to which they respond best. Some like bluer light in that 400nm range, some like a more red light in that 600nm range.

Now the PAR you chose to light these corals with may be identical, for arguments sake 300. But the Spectrum in which makes them happiest will be different this is PUR.

In the ocean depth also affects spectrum, red wavelengths are filtered first. The bluer the spectrum and closer to 450nm the deeper into the Reef that light will penetrate.

If we go back to the human diet analogy we know that we need to look at calories and food types but we also know one size doesn’t suit all. The world’s strongest man needs a different diet to your average office worker, both diets are right for those individuals but not if they are interchanged.

To assume all coral need the same light would be to make the mistake. PUR gives us away of understanding what a specific coral needs. There are some general PUR lines that can be used but as we learn more new PUR lines will be drawn. So, giving corals plenty of PAR is all well and good, but we also need to ensure its PAR at the correct PUR.

Now taking this one final step further, whilst remembering i am no biologist and most hobbyists will not care or need to research this deep.

Our corals are made up of minute groups of cells named Zooxanthellae that live symbiotically inside the coral. These zooxanthellae are divided into groups called clades, each clade requiring a different spectrum of light (PUR).  So far 8 clades have been identified and they are made up of a mixture of cells including photosynthetic cells and proteins. It’s these proteins that make some corals glow. These corals have evolved these proteins for two roles; some for protection from UV and some for producing wavelength that are   A coral can have more than one clade and in differing amounts in the same coral species depending on location and conditions; this is called photo adaption. So even though a coral can adapt slightly to different light they are limited by the cells types in their clades.


Some of the common photosynthetic cells (which I will mention in a moment) are the work horses of the coral and where most energy for coral growth comes from due to light hitting them and not food added to the tank. They are:

Chlorophyll A, one of the main photosynthetic pigments. It absorbs light best at 2 points, a peak of 430nm (blue) & 660nm (red)

Chlorophyll B, another of the main photosynthetic pigments. It absorbs light best at 450nm (blue) & 640nm (red)

Carotenoids are another key group of pigments that absorb violet and blue light typically between 400-500nm.

In photosynthesis, carotenoids help capture light, but they also have an important role in getting rid of excess light. When a coral is exposed to intense light it receives a huge amount of energy. If that energy is not handled properly, it can damage the coral and cause zooxanthellae to expel. Carotenoids help absorb the excess energy and dissipate it thus protecting the coral.

So now we know that our corals need a specific range of PAR and a particular Spectrum and we also know that devices like the Seneye Reef or an Apogee light meter may help us to measure these. There are new products coming to market that can see all the wavelengths of the lights spectrum and PAR and therefore tell us our PAR levels based on different corals needs, one of these will be the upcoming Seneye Spectra something we will look at in a future video.

So now with all of this knowledge in hand, how do we know which is the best or the ‘right’ form of light for our reef tanks?

There are many forms of lighting on the market today. LED, T5, metal halide but to name the most popular.

About as far back as I can remember metal halide was the ‘goto’ method of lighting for our reef tanks. This then, with technology, advanced to T8 tubes, then T5 tubes and now LED.

Many hobbyists today still favour Metal Halide and T5 lighting over LED due to their ability to ‘just work’ and for corals within their tanks to grow. However I believe there are still many issues here.

Both of these forms of lighting are relatively inexpensive to purchase but are power hungry. They use a lot of energy and thus get warm. They can create unfavourable ambient temperatures and produce problems in your tank. They are also becoming more difficult to dispose of due to the chemicals contained within the tubes and their requirement to be replaced every 6-9 months due to bulb degradation and spectral change makes them expensive long term.

Where T5,T8 and MH are favoured however is their ability to work out of the box. The reason for this is manufacturers design these bulbs and tubes at a specific PAR and Spectrum and the user has no ability to tinker and adjust them. They were tested in their respective labs and all a user has to do is install at the correct height and they have the correct PAR already achieved for them.

The advancement of technology has seen LEDs introduced to aquarium lighting and for a number of reasons; I believe they are significantly better for our aquariums than all previous iterations of lighting.

The units are more expensive initially and you will likely require more lighting units to achieve the same level of spread and provide the same even coverage as with previous lighting. Buyers should also be aware that not all LED lights are made equally, they are complicated to manufacture correctly and their design needs to clever to be energy efficient. A comparison of an LED’s light output by watts it consumes is not relevant at all. PAR should be used as the minimum guide to efficiency. Some LED lights may perform at a high wattage due to inefficient cooling of the unit, others may more be more efficient and thus require less energy to work effectively. What I’m trying to say really is don’t assume more watts mean more light.

LED has the great advantage (or disadvantage deepening on who you are) that a user can customise and ‘edit’ the lights parameters. We can adjust single LED colour channels, i.e turn the white up and blue down (or vice versa). This in turn gives us so much control and it could be said in most cases hobbyists will simply guess what values are best based upon the view to their naked eye without any testing or research.  Hobbyists are then quick to say that a particular unit doesn’t work as their corals don’t grow OR worse, their corals are dead. Users unbeknowingly may be increasing PAR to dangerous levels or be lighting their tanks with insignificant PAR in the first place.

Now these same hobbyists likely have never ever taken a PAR or PUR meter to their aquariums and measured these levels and have used guess work to reach a conclusion. Their previous light, an old T5 or Halide unit worked well, so why don’t these new LEDs?

It is so important to understand the information i shared earlier in this video and therefore understand what each coral needs and why it is important to test both PAR and PUR to reach this conclusion.

The more advanced and reputable brands of LED lighting available on the market at the time of filming this video like Ecotech and AI with the Radion and Hydra range would always get my vote but it’s also important to understand that when applying settings or another users schedule you know what your applying, how that was created and what the user had in mind when creating it. Selecting a preset designed for a 4ft deep tank in your 1ft deep cube and wondering why it doesn’t work is fairly obvious when you think about what we have discussed so far. The likes of Ecotech have produced some really stellar research as part of their Coral Lab programme and US based BRS TV have also produced some incredibly valuable videos on lighting well worth a watch.

The blending of the spectrum of light is also very important in lighting units, its all well and good hitting a spectrum, achieveing ‘xyz’ PAR but if the unit cant blend that light into a uniformed dispersed area youll be left with a disco ball affect, one area being more red and another more blue etc. More capable lights like the Hydra/Prime & Radion are diffused to better blend the lights and reduce those negative affects but many cheaper brands of LED are not.

You also need to ensure that PAR is measured at various points of the aquarium. It’s all well and good to say that the PAR is ‘xyz’ level at 12’’ below the water surface, but what is it up high, what is it down low, and what is it on the edges of the tank? More LED units may be required to achieve that uniform even level of coverage and ensure your tank is lit correctly.

It’s also worth pointing out not all units are equal. Every manufacturer should list output information on their websites or the box giving you maximum levels of PAR or telling you what the lights kelvin or lux may be, ideally they will tell you all of the above. Don’t be fooled into buying into lights for a set of features that are irrelevant to the overall performance of the light. Manufacturers are now developing this sense of a need to ensure lights are feature rich and forgetting whats actually important. Equally not all manufacturers use high quality components capable of achieving certain wavelengths, missing some entirely that are needed for photosynthesis.

My personal recommendation through personal use as of today would be Ecotechs Radion line or AI’s Hydra and Prime HD line.

Now we know more about what lighting is, how it works and what our reef tanks need, in our next video we are going to take a look at HOW we measure lighting in our reef tanks, where we should look to measure from and ultimately what those results mean.

If you want to learn more OR you dont want to read all of that again, fear not, i filmed it!

As a lifelong adult hobbyist, making it almost 15 years in the making, Danny has been keeping Saltwater for quite some time. As one of the biggest passions in his life in 2014 he combined that with his second biggest passion, photography and videography.