Do You Have To Have Metal Halide Lighting For Hard Corals?

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.

---

Interesting Reading From Across The Blogosphere

Lighting is Complicated
- There are, of course, many different types of coral. At the lowest levels of classification, corals generally fall into two categories: hard or soft. Hard corals come in two varieties, large polyped stony corals (LPS) and small polyped …

How To Change A Fish Only Aquarium Into A Reef Aquarium
- The blue tubes stay on all the time from switch on to switch off, this is because they are not just there to create the ‘dusk/dawn’ effect, but they assist the corals as their light spectrum suits the zooanthellae algae in the coral …

---

Zooxanthellae

The first thing that needs to be done after writing the above is to ensure the spelling is correct. It is a strange word to us ordinary mortals, but came into being with marine biologists.

In the marine world one of the most fascinating double acts to watch is that of a clownfish and an anemone. The clown swims to the anemone and gains protection from the stinging tentacles, and, though there is argument, the anemone may gain by the clown chasing off unwanted threats to the anemone or dropping bits of food on it. To my knowledge anyway the anemone’s advantage is not fully clear.

The above example illustrates how two different life forms can be of use to one another. There is another that is much more common in marine aquariums. This one is the relationship between corals and zooxanthellae. Zooaxanthellae are in fact single celled algae that live within the tissue of a coral, often called symbiotic algae. Why should this be?

It is thought that the coral uses the oxygen and carbohydrates manufactured by the zooxanthellae. In turn, the zooxanthellae utilise coral waste products and assist in obtaining trace elements from the seawater. It is theorised that up to 90% of the corals required food may be supplied by the zooxanthellae.

For the algae light is very important, as it is required for the photosynthesis process. This is why reef aquariums are brightly lit, some with T5 fluorescent tubes and others with metal halide bulbs. Power output and spectrum need to be correct (power output for the depth of the aquarium). It is thought that the ‘actinic’ fluorescent tube is very beneficial to the zooxanthellae.

A clue as to whether a coral has zooxanthellae or not is in the colour. Generally, it is likely that corals coloured yellow, purple, red, and orange do not have the algae, and corals coloured blue, beige, brown and green do.

Both soft and hard corals may have zooxanthellae. In addition, there are others, such as the aforementioned anemones and clams such as Tridacna.

Problems can arise with this strange relationship but this is not the text to discuss that. Provided the aquarist provides adequate lighting and water quality, then all should be well.

(Reference: Marine Invertebrates. Martin Haywood and Sue Wells)

---

Aquarium Additives and Hard Corals

The reef aquarium with hard corals ([tag-tec]SPS[/tag-tec]) is the desire of many an aquarist. The sight of pictures on the internet and in books only serves to increase that desire. With modern aquaria and equipment, plus the knowledge gained over recent years, this desire can be met, and more easily than some suspect.

It is taken as read that the need for high water quality is understood. High water quality means a proper and stable pH, between 8.0 and 8.4, nitrate less than 10 ppm (parts per million), phosphate preferably undetectable, but no more than 0.03 ppm, and ammonia and nitrite undetectable. Equipment should include a properly sized and efficient protein skimmer. Water movement, which I consider a part of [tag-tec]aquarium water quality[/tag-tec], should be vigorous and chaotic. There is also a need for strong lighting, the best of which is metal halide, supplemented by actinic T5 fluorescents. A greater number of T5 fluorescents can be used without a metal halide, as many as can be fitted into the aquarium including their reflectors. The T5′s should be an equal, or as near as possible equal, mix of marine white and actinic. However, at least in my opinion, the halide lighting is the best option.

If the reef is a mix of hard and soft corals, then parameters should be for the needs of hard corals. Soft corals will not suffer. Water quality measurements (pH, nitrate, phosphate etc) would be the same for both.

So that’s sorted then, back to additives.

Hard corals require calcium, and require it considerably more than the soft corals. Modern commercial salt mixes contain a good level of calcium, particularly those more recent ones which are specifically designed to give a high calcium reading for the [tag-tec]reef tank[/tag-tec]. The calcium level that should be maintained is 450 ppm, and should not be maintained at less than 400 ppm. (Some aquarists have the level at 480 ppm or sometimes a little higher.) This does not mean to say that the calcium reading must always remain exact at a particular level, it will of course vary to a reasonable extent. Even with regular routine water changes, a hard coral reef ‘s demand will reduce it.

So how can calcium be maintained. The first option is supplementation with commercial products. The product instructions advise how much should be used to raise the calcium reading by X, according to the net gallonage of the aquarium. All increases should be gradual, and the instructions followed. These products are suitable for a small aquarium, for instance a nano system or up to a net fifty gallons or so. The products are not cheap, however, and larger aquariums will require larger doses so other methods need to be considered. The first, and recommended method, is to purchase a calcium reactor. This is an external device where the aquarium water is passed through the device, and is exposed to a chamber containing a calcium carbonate medium. This would normally have little effect. However, carbon dioxide is also fed into the chamber, and this causes the medium to slowly dissolve. As it dissolves it releases calcium, along with other trace elements. It is an efficient way of maintaining calcium. The water flow rate and the input of carbon dioxide must be correct and controlled. The aquarist must change the medium when it appears ‘mushy’ or is nearly gone. The calcium level must also be tested at least weekly as it is important to know that the reading is that desired. If too much calcium is being produced, then the running time of the device can be reduced until correct (ie. turned off for X hours each day). There are other means of introducing calcium, for example kalkwasser (limewater). Kalkwasser has drawbacks: it should really be mixed fresh each day which is a fussy procedure, and, as it has a high pH, needs to be introduced to the display aquarium slowly and carefully. The aquarist needs to exercise care as the mix is caustic. In addition, if the calcium level in the aquarium is low, the kalkwasser may be ineffective because the amount that can be introduced to the aquarium is limited. Put too much in and the sudden pH rise could be dangerous to the reef.

Talking of calcium brings the discussion to alkalinity. This is an important parameter and should be within certain readings. These reading are 9 to 11 dKH (151.1 to 196.9 mg/l). The alkalinity of seawater is its ability to resist downward changes in pH. It has a buffering capacity. Hard corals require stability and ensuring that alkalinity is at an acceptable level greatly assists this stability. Alkalinity affects the amount of calcium (and trace elements) that can be maintained. However, if alkalinity rises too high then calcium levels that can be maintained will be lower. So, as said, alkalinity should be maintained at a reading as suggested. How is the reading maintained? Again, commercial preparations are available which contain carbonates and bicarbonates. These should be mixed and applied as per the instructions supplied and according to the net gallonage of the aquarium. Again, the commercial preparations may prove to be too expensive for large reef systems. There is an alternative, applicable to small systems as well. Purchase some ordinary baking soda (sodium bicarbonate) from a shop. If this generates too much nervousness, obtain it from a chemist. It is safe but should be dosed carefully. As there will not be any instructions for use with a marine aquarium the following is a guideline: when raising alkalinity, divide the aquarium net gallonage by 25. Add a teaspoon for each 25 gallons. So 50 gallons would need two teaspoons. Aquariums with a net gallonage lower than 25 would need to add the appropriate fraction of a teaspoon. Mix the soda in some aquarium seawater and add to the sump, or to the aquarium away from livestock. That is the full daily dose, the alkalinity level should be checked by testing before adding any more on the following day. Once the required level is reached, the aquarist will soon discover how much his/her aquarium needs to be supplemented. For alkalinity and calcium, the following link gives further information. It really relates to those who are experiencing problems, but nevertheless gives information and parameter suggestions.

http://www.advancedaquarist.com/issues/nov2002/chem.htm

Iodine is considered by many to be beneficial to corals. This often refers to the soft variety, but there is a fairly strong body of opinion that suggests hard corals will benefit as well. Iodine in natural seawater has a very low presence, 0.06 ppm. Commercial preparations are available, and if used regard must be given to the net gallonage of the aquarium and the manufacturer’s instructions as usual. There is a drawback, and this is that, as far as I know, there isn’t a test kit that will reasonably accurately check iodine levels. If that remains the case, then do not exceed the recommended dose if the aquarist must use it. Alternatively, if regular routine water changes are completed and if the corals are healthy, expanded and growing, then don’t use it.

There are all sorts of trace element additives available to the aquarist. For example, strontium, molybdenum and iron. Other available magic bottles are said to contain a mix of essential elements. I am not a scientist. Nevertheless, the books I have read by very knowledgeable and up to date authors suggest that there is no scientifically supported need for these to be supplemented, therefore it must be that any benefit suggested from their use is anecdotal. I do not, and am not qualified, to challenge their use. However, I would suggest doing regular routine water changes using a high quality mix. There are several good preparations on the market nowadays. These will introduce trace elements to the aquarium seawater. Observation of the corals will advise the aquarist accurately: if the corals are expanded and healthy then.fine