Many reef aquarists see hard corals as the height of achievement and the closest thing to a real coral reef in captivity. This isn’t quite true of course, as parts of coral reefs are dominated by soft corals. Nevertheless, a hard coral reef can be very beautiful, and if the aquarist has selected the corals for shape and colour carefully it makes a stunning picture.
The very first requirement for a successful hard coral reef is seawater quality – it must be high and consistently so. This includes the necessary levels of calcium, alkalinity etc. Water movement must also be appropriate, and hard corals generally prefer considerable movement, more so, again in general, than the soft corals.
The second requirement is appropriate lighting. This should be of the appropriate power, which is measured in watts (W) and also the appropriate spectrum. With spectrum, metal halide bulbs are usually purchased with a Kelvin (K) rating. Kelvin is a measurement of colour temperature. The perceived colour of the bulb will become more blue and colder looking as the Kelvin rating increases. For example, a 6500K bulb will appear more yellow than a 20000K bulb, which will appear more blue, or colder looking. Nowadays, it seems there is favour with the 14000K bulb, though the best Kelvin rating for corals is still argued over.
It should be remembered that the corals harbour symbiotic algae known as zooxanthellae. This algae is important to the corals so it is necessary for it to thrive. Zooxanthellae need light of the correct type so that they can photosynthesise. Current thinking is that blue light of the correct wavelength is needed. This seems reasonable, as much light of other colours is lost as the depth of the sea increases. Blue is nearly the last remaining light at depth and must therefore be available to the zooxanthellae in the natural habitat.
So, getting back to the point. Excluding power LED’s (now becoming more available) which are very expensive, there is really only one other light type that could be useful with hard corals, and that is the T5 fluorescent tube. These tubes come in various spectrum outputs, and the two that are of most interest are the marine white and actinic blue types. They should be fitted with reflectors and as many as possible placed over the aquarium, with an equal number of actinic and white if possible.
T5 tubes can produce the correct spectrum, but there is another problem, and that is light penetration. To be fairly sure that hard corals will be adequately lit, the tubes need to be fitted as close as possible to the water surface, maybe two or three inches away. This is quite safe with the fittings used nowadays. Even so, light penetration may not be adequate to any depth, and the corals could need to be sited in the top third of the aquarium (the top third being subject to the overall aquarium depth of course). Some adjustment of the coral’s position could well be required as time progresses.
It is feasible to have a reef such as described in the previous paragraph, with corals that are happy with less light lower down. If the aquarist selects carefully, then many soft corals could be used, but it is necessary to check that these corals are not going to be detrimental to the hard corals. This is because some corals use weapons such as sweeper tentacles which sting, and some soft corals use chemical warfare.
The metal halide bulb has more power (W) and therefore more penetrative ability. The light produced reaches corals lower down. There are guidelines on the power needed for different depths, to provide light for light loving corals. These are guidelines, not rules:
150W 10″ (circa 25.5cm)
250W 14″ (circa 35.5cm)
400W 22″ (circa 56cm)
While we’re at it, another guideline – a metal halide bulb can light around 36″ (circa 91.5cm) of aquarium, that is, 18″ (circa 45.75cm) each side of its centre. So a 72″ (circa 183cm) aquarium could need two bulbs placed 18″ (circa 45.75cm) in from each end. Again, these are guidelines.
As can be seen, ideally the depth of tank needs to be taken into consideration in the planning stage. There are other factors that can affect light penetration but the above is a general guide. It does not mean that a coral will not grow below the stated penetration point – the growth rate could be slower or the coral could be selected by the aquarist to suit the light conditions. (We’re back to that word again – research.)
Some aquarists fit, and many manufacturers provide, actinic fluorescent tubes to be used with the metal halides. Many aquarists consider it advantageous to do this. There is argument over the necessity of it – what if the Kelvin rating of the bulb is high and into the blue range? However, to my knowledge anyway, the additional fluorescent tubes don’t do any harm and seem beneficial.
There is another point that needs to be mentioned about metal halides, and the word ‘point’ is the point. (Err, I’ll tie myself up soon!) Because a metal halide bulb is a point source for light (a fluorescent tube emits light over its full length), a lovely ripple effect is created within the aquarium, much like the sun would create on the wild reef. There is argument about the importance of this among advanced aquarists, but it is enough to say that it is an advantageous effect, at least it is to me.
Two important disadvantages of metal halides are first, that they are expensive to run, and second, they can overheat the aquarium water. So before a final decision is made, consideration must be given to the cost in electricity and also to the environment the aquarium is subject to. If it is a naturally warm environment, there could be a need to invest in a chiller (seawater cooler) which in itself is quite expensive to purchase and fairly expensive to run.
Overall, the metal halide is the light of choice for a hard coral reef at the moment. As said, fluorescent tubes can be used, with extra care exercised by the aquarist.
Anyone planning a reef aquarium should be aware that the more than probable reef light of the future is the power LED array. This is expensive to purchase at the moment, but the aquarist may wish to check on the current situation.
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