Special Lights On A Reef Aquarium – Why?

“The fish are fine under standard non-aquarium lights, even if I buy just white lights they are still ok. Why not corals?”

This is an understandable question. Marine reef fish come from the same areas as the corals so why spend more? There is a good answer and no, it isn’t just a good way of making money!

First let’s make a comment about ‘just white lights’. This can mean any kind of bulb. Marine fish will live happily under ‘just white lights’ as they need to see and basically that’s it. However, lights designated for marine aquarium use (for fish only this is usually fluorescent) should not encourage unwanted algae. Lights that have an unwanted spectrum particularly in the red area can. So two tubes, one marine blue and one marine white will be better. Two tubes allow for a ‘dawn/dusk’ sequence (using two timers, see below) and with some fish the light output enhances the fish colours.

So what about corals? With fish it can be an advantage to use blue and white as above. With corals it is essential. Why? Within the flesh of light loving corals are small single celled algae called zooxanthellae. These cells are very tiny, a square millimetre could hold about 10,000! They live as tenants, the rent is nourishment for the corals. The zooxanthellae can supply between 60 and 150% of the coral’s energy requirement*.

How do the zooxanthellae obtain the nourishment for the corals? In the same way as living plants, including visible algae do – from the sun. This is known as photosynthesis. Corals having ‘tenants’ possibly started because tropical reef waters are nutrient poor.

Now we need to go a little further. We’ve mentioned marine white and blue as needed colours, this is because light is lost in seawater and different colours are lost at different depths. Some corals live near the surface, others live deeper down. High penetration of seawater is achieved by blue light and it is known that zooxanthellae are able to make good use of this light. The blue light gives a lovely colour to the aquarium picture on its own, but the light is balanced by the white light. Also, as with fish only, the aquarist can create a ‘dawn/dusk’ sequence. The sudden application or withdrawal of light isn’t good, so the blue lights can come on say 15 minutes before the white (dawn) and reverse at the end of the day (dusk). The overall message is, for success with corals the lighting should be correct.

There are choices as usual and technology advances all the time. However, in my view simplicity isn’t a bad thing and so ‘straightforward’ lighting is required.


First, there are fluorescents which are readily available and suitable. Ready made lighting canopies can be obtained containing more than one fluorescent but let’s keep costs down and buy single tubes. Check the length of the aquarium and make sure the ones we choose will fit. Consider reflectors for the tubes and obtain the maximum that, put in lengthwise, will fit across the aquarium. The number of reflectors dictates the number of tubes of course. Obtain marine blue and marine white tubes, if the reflector number is even make one half blue and one half white. If the number is odd make the extra one white. Now it is easy to get the correct power units – these can be obtained so that two tubes run off one so creating more simplicity. Two electric timers are needed for the ‘dawn/dusk’ sequence. The supplied power connectors for the end of the tubes are specially designed so are safe if the reflectors/tubes sit on the glass cross bars (but does not remove the need for caution when electricity is close to seawater!). The usual tubes used are called T5’s. Fluorescent lighting is reasonably cheap to run and does not emit excessive heat. The tubes need to be changed at the latest once a year, probably better at 6 to 9 months.

So that’s simple and not a tremendous expense. However (oh yes, here it comes!) tubes cannot be used on any reef – it depends on the depth. The light has to reach the corals with sufficient power so research and/or advice is needed. There’s plenty of information available, including on this site.


Another popular lighting system is called metal halide. These come in prepared packages though DIY is available. Dependant on the length of the aquarium, one or more light units are hung above, often one for every 3 feet, not too close to avoid them being splashed (this is because the bulbs become very hot). The light unit(s) are connected to controllers which could have timers. The bulbs available are designed for marine use and come in various light types some being more white and others more blueish in appearance. Some aquarists make use of one or two marine blue fluorescent tubes as well, so the zooxanthellae are happy and a ‘dawn/dusk’ sequence can be achieved. The metal halide system is a very good lighting system for deep reef aquariums as light penetration is very good. Different power level bulbs are available, two being 250W (watts) and 400W. Reading those wattages gives a clue to the downsides, the first being they are expensive to run, more so the more bulbs there are. The second downside is the heat the bulbs emit which can quite easily heat up the seawater. This additional heat could be a real problem in summer when natural heat is higher as the seawater temperature increases – the heater(s) fitted will of course switch off but the temperature could continue to increase, even to the point where aquarium life is stressed or threatened. To resolve this problem could mean the purchase of more equipment, a cooler. So again research and/or advice is required. With the lights on between 8 to 12 hours a day electricity costs are going to be very noticeable, plus the possibility of additional problems as mentioned. The bulbs too will need replacing at generally the same periods as fluorescent lights.


Finally, there is now what appears to be an excellent choice of lighting. This is the LED system. Basically blue and white LED bulbs are mixed to give good coverage of the aquarium. The lights can be a DIY project, or purchased as an array that covers a large area of the aquarium surface. Alternatively, LED strips can be obtained, these are similar to fluorescent tubes and reflectors in size and are fitted in the same way though the blue and white is present in each strip. Though things are improving all the time the disadvantage of these systems is the high cost, particularly the ready built full array. The strips are not as bad though several are required. There are some definite advantages though. The first is the very long expected life of the LED bulbs which could be up to 20 years! The second is the low running costs – obviously they still need electricity but nothing near the demands of a metal halide system. The third is the heat output, it isn’t going to be a problem and will not normally affect the seawater. In some systems there is an in-built fan unit that directs any heat generated away from the aquarium. Another ‘advantage’, and this depends on how the aquarist views it, is the ability for example to simulate clouds passing over the reef. Most aquarists would probably view this as a non-essential high level luxury and fair enough. It is available in or with some full canopy arrays and will add to the already high purchase cost.


So special lighting on a reef aquarium is essential. It is of course up to the aquarist to decide which type, the decision will effect the later costs of running the system and the cost of buying it in the first place.

One thing is for sure. If the reef system is to be a success, if the aquarium is going to be as beautiful and interesting as it has the potential to be whatever its size, and if the aquarist is to meet the ongoing cost of running the system, the correct choices have to be made. So research and advice, then consideration. It’s worth it!

(*Reference: Eric H. Borneman. “Corals”)