Keeping Corals: Fact or Fiction?
by J. Charles Delbeek M.Sc.

Since the advent of miniature reef aquariums (Mini-Reefs), the amount
of interest in keeping corals in a tank has increased greatly. In
this paper I would like to briefly discuss the 2 basic groups of
coral followed by a discussion on aquarium requirements such as water
quality, lighting and nutrition. I won't go into great detail on any
one thing, I'd just like to peak your interest enough to cause some
questions to start coalescing in your brain.

There are basically two types of coral groups: hard corals and soft
corals. Hard corals have hard, external skeletons composed of
calcium carbonate. The polyps are located in individual compartments
called coralites and these are the openings one often sees on coral
skeletons. The polyps of hard corals have 6 or more tentacles but
never 8. Polyp size can range from a few millimeters (eg. Porites
sp.) to several centimeters (eg. Fungia sp., Catalaphyllia and
Euphyllia sp.). In contrast, soft corals lack an external calcareous
skeleton and have, instead, tiny spicules of calcium carbonate
embedded in a soft tissue; these spicules are used in identification
of the various species. Soft coral polyps always have 8 tentacles,
hence their common name of octocorals. Although octocorals are
usually soft and pliable, there are octocorals which can be
surprisingly hard such as Red Organ Pipe Coral (Tubipora musica) and
Blue Coral (Heliopora coerulea).

In the past, corals were infrequently imported or carried by local
retailers. The common Orange Tube Coral, a.k.a. Sun Coral (Tubastrea
aurea) and Goniopora sp. were among the more common varieties
imported and success by the average hobbyist with them was limited.
Many of the beautiful soft corals such as mushroom anemones (Ricordea
sp., Actinodiscus sp. etc.), Leather corals (Sarcophyton sp.,
Lobophyton sp.), soft branching forms (Sinularia sp., Litophyton sp.,
Cladiella sp., Alcyonium sp.), encrusting Star Polyps (Pachyclavularia sp.
and Cornularia sp.). gorgonians (Eunicea sp., Briareum sp. and
Pseudptergorgia sp.) and pulsating encrusting polyps such as Xenia
sp. and Anthelia sp. were unheard of. In the last four years,
however, these corals have become very popular and some will even
reproduce and grow in aquariums. Others, however, waste away through
either neglect or lack of knowledge of their requirements.

Aquarium Requirements
1. Water Quality
The range of water quality that corals can be kept under can vary
depending on the particular genus we are talking about. Certain types
such as mushroom anemones and leather corals appear to be more
tolerant of water quality than other forms especially the reef
building hard corals. The two main culprits appear to be nitrate and
phosphate. When attempting to keep hard corals and most soft corals,
nitrate and phosphate levels should be kept as low as possible,
preferably below 5.0 and 1.0 mg/L (ppm) respectively. The reason
being that both tend to inhibit calcification, especially phosphate.
Reef levels of both these nutrients are generally less than 0.01 parts
per billion (ppb/ug/L) so you can see that our tanks are literally
nutrient soups compared to the reef. However, some corals CAN be
found in turbid areas with higher nutrient levels such as near
harbours or river outlets but the species diversity is quite low.
Usually what one sees, as pollution/turbidity increases, is a
preponderance of leather corals and mushroom anemones and a decrease
in the numbers and variety of hard corals (Wilkens, 1987). Even in
seemingly identical habitats the coral populations can be quite
different.

Temperature is an important factor in keeping aquariums in so far as
it must be kept above 72 degrees F and below about 80 degrees F.
True many reefs of the Caribbean and Pacific can reach higher levels
but in an aquarium, this places unnecessary stress on the animals.
What many people don't realize about the temperature though, is that
it is not as steady as widely believed. On areas of the reef flat,
low tide conditions can bring about greatly increased temperatures of
over 90 degrees F. This is probably why large shallow-water Leather
corals do so well in aquariums; the are more tolerant of changes in
water quality. Many corals also spawn in relationship to seasonal
water temperature changes and light level changes. The affect
(indeed the significance if any) of removing this stimulus, as occurs
in closed systems, has not, to my knowledge, been investigated.

Dissolved oxygen content is another important factor; too little and
the coral may suffer, too much and the same can occur. If you have
alot of algal growth in your tank such that bubbles appear on the
surface of the algae, your oxygen level is likely at the saturation
point. Excess algae can easily drive the water to supersaturation of
oxygen. If similar bubbles begin to appear on your coral polyps,
then the zooxanthellae are manufacturing excess oxygen (hyperoxic
conditions) and this can lead to damage/death of the coral; oxygen is
poisonous at high concentrations.

Much has been written about the addition of trace elements for reef
aquariums. However, I am skeptical as to the validity of the
advertisers claims about what is needed and by how much. Peter
Wilkens, a German hobbyist, has shown over many years of
experimentation that certain elements appear to be necessary for the
continued maintenance of corals. The primary one is calcium; not
surprising since calcium is the major building bloc of both hard and
soft corals. The regular addition of a saturated calcium hydroxide
solution is highly recommended by numerous European authors who
report excellent success with corals since using this method. The
other element that Wilkens maintains is necessary is strontium,
claiming it is involved in the laying down of calcium carbonate. One
thing I have noticed is that the KH of aquarium water drops very
little as Ca(OH)2 solution is added as top-off water and the growth
of calcareous coralline algae definitely increases. I will soon
begin adding a strontium chloride solution to see what additional
affects this may have.

II: Water Movement
Since corals are immobile creatures they need the water to both bring
things to them as well as take things away. It is for this reason
that water circulation is one of the key ingredients in maintaining a
reef tank. I'm not talking here about the rate of flow through the
tank and filter but of the movement of water around the tank. Water
flow is extremely important to corals because it carries dissolved
gases and food to them and removes wastes. Even in weak to moderate
current flows, a micro layer of still water can exist around a coral
that may cause it to suffocate in it's own wastes. Good water flow
also stimulates coral growth and can cause their polyps to open
more. Again, the degree of water flow is often species specific but
the majority require medium flow with occasional strong bursts.
Some, such as mushroom anemones, like a very gentle flow while other
such as leather corals, photosynthetic gorgonians and star polyps
enjoy strong flows. It is important to realize that on a reef, water
flow is rarely constant in one direction. An oscillating (back and
forth) flow is much more common and anything you can do to give your
corals this type of movement is an improvement. The other thing
about an oscillating water flow that is especially important to those
corals that have zooxanthellae, is that it exposes more polyps to the
light.

III: Lighting
This is probably the one topic about which the most arguments exist:
which type of lighting is best to use for reef tanks? I imagine you
think that I'm going to tell you which is the best right? Nope ...
wrong! There is no "best" type of lighting simply because all
corals, unlike man, were not created equal. The problem lies in the
fact that there is a great deal of variability within one species of
coral. Both coral shape and colour can vary drastically depending on
what depth they live at or under what light conditions (i.e. shade
vs. light) they develop under. This results in two specimens that
may look completely different and subsequently have different
lighting requirements, yet belong to the same species. It is this
fact that has made coral taxonomy historically difficult and has
posed numerous problems for aquarists.

The choice of lighting to use contains numerous variables that must
be taken into consideration. The first of these is the type of
invertebrates you wish to keep. If you would like to keep
invertebrates that rely on their symbiotic algae (zooxanthellae) for
their nutrition, then light intensity and spectrum are important.
If, on the other hand, you want to keep invertebrates that lack
zooxanthellae such as Dendronephthya sp soft corals, Orange Sun
Corals, Tubastrea aurea or certain gorgonians, the lighting is less
critical. Another variable is the natural habitat that the coral was
collected from. Deeper water corals (30 ft.) do not require the
intensity of light that shallow water forms require and do not need a
complete spectrum of light to do well. Shallow water (ft.) forms
require greater intensity and a wider spectrum of light than other
forms. The greatest decrease occurs in the red end of the spectrum
within the first 10 m (30 ft) with maximum transmission occurring at
a wavelength of 480nm (blue light). Intensity drops off dramatically
with depth too (see Moe, 1989). I believe that it is these facts
which have sparked the intensity vs. spectrum debate amongst
hobbyists. Hobbyists don't seem to take this into consideration when
purchasing and placing new corals in aquariums. They may put newly
acquired pieces directly under their metal halides and watch them
open up wide for a time then slowly wither away. Transplant studies
have shown that corals taken from deep water and placed in shallow
water die-off rather quickly while those moved to deeper water from
shallow water, grow more slowly and change form (Dustan, 1982). We
will go into why this occurs in a moment.

Lets first take a look at the two main forms of lighting available:
metal halide (HQI) and fluorescent. Before I begin I would like it
made clear that I am not endorsing one form of lighting over
another. I will simply supply you with the information that I have
obtained from various sources and my own experience. Take this
information, read whatever else you can get your hands on and then
make your own decisions. Forms of lighting that are definitely not
recommended for reef tanks are mercury vapour and sodium vapour
lights as well as HQL and HQI-NDL lighting which have colour
temperatures (4300K) and spectrums that are unsuitable.

Metal halide lights have been blamed for many things, much of which I
think can be traced to four factors: improper bulb choice and
placement, inadequate shielding and poor specimen placement. The
choice of bulb is important because many of the HQI lights sold early
on had colour temperatures of 5000K or less which lacked sufficient
blue and had too much yellow and/or red. The spectrum of these bulbs
was not really suitable for reef aquariums but were often recommended
by those who considered intensity to be more important than
spectrum. For example the aquarium in Puerto Rico discussed by
Julian Sprung (1989) used 4300K lamps which may have been the reason
for the decline in that aquarium's corals. Other bulbs (i.e. Energy
Savers 5500 K and Osram D 5200 K which are used in some Dupla
fixtures) have higher temperatures (5200 K) and provide more blue.
Although some (Thiel, 1988) maintain that HQI lamps alone give enough
blue I don't feel this is true; even better success can be achieved
by using a pair of HO or VHO actinic bulbs in conjunction with the
HQI. It has been shown that blue and white light promote greater
skeletal growth in hard corals, and their isolated zooxanthellae,
than green or red light (Kinzie et al, 1984). Therefore, additional
blue light will only have beneficial results and should be favoured
over the addition of higher intensity lights that have more red or
green light.

Wattage is important too. Obviously one must take into consideration
the type of invertebrates one wishes to keep, at what depth they
normally occur and the size of the tank before selecting the wattage
of bulb to acquire. Another factor might be the degree of heating
that the light will cause in the tank; metal halide lights should be
at least one foot above the water surface to prevent overheating of
the water. Some European aquarists recommend that one 250 W HQI
lamp, at a height of 35 in. (90 cm) above the bottom of the tank, is
sufficient for a length of 35 in. (130 cm )(D. Stüber, personal
communication). I can't help but think that many recommendations on
the amount of light required have focused on studies of light
intensities found at a depth of a few metres (see Sprung 1988). What
has failed to be taken into consideration are the following facts.
First of all not all our aquarium specimens come from such shallow
areas and have no need of such high intensities. Secondly, even in
these shallows, the light intensity is far from constant. Light
intensity gradually increases over the day, peaking between 1100 and
1400 hours, after which it gradually decreases again. Therefore,
high intensity light is only present for a few hours per day.
Thirdly, the affects of clouds and weather greatly reduce the amount
of light that eventually reaches the water surface, therefore the
number of hours and days when the reef actually receives the maximum
amount of light available is actually quite small. Wilkens and
Birkholz (1986) recorded the lux readings at a depth of 1 m on a reef
in Indonesia. They found that values ranged from a low of 2800 lux
in the morning to 14 000 lux by 1100 hrs. and fluctuated after that
point, due to intermittent sun and cloud, between values of 1700 and
22 000 lux, with peaks of 26 000 lux from noon till 1400 hrs.,
falling quickly again in the late afternoon to values between 9 000
and 7 200 lux. These values would be strongly attenuated with depth
and suspended particles in the water caused by the turbulence from
waves and tides, such that intensities will drop off drastically
within the first 5 to 10 metres (see Dustan, 1982). In the
relatively particulate free, low dissolved organic carbon content
water that exists in properly cared for aquariums, the amount of
light fall-off may be negligible. However, measurements by Thiel
(1989) and Burelson (personal communication) have shown that a
significant drop-off in intensity actually does exist, even in
relatively shallow aquariums. This must be taken into consideration
when deciding upon the size of bulb to obtain and the overall design
concept of the aquarium (i.e. types of organisms and their
placement). Finally, the greatest variety of coral growth occurs at
depths between 30 and 40 ft. (10-15 metres) where light intensities
are much lower than 20 000 lux (see Dustan, 1982).

As has been reported elsewhere, HQI lighting produces ultraviolet
(U.V.) light which can be harmful to those corals that lack
appropriate U.V. shielding pigments (Mohan, 1990). This can be due
to the fact that they were collected from deeper water or that they
have lost these pigments during shipment/captivity. Add to this the
fact that many fixtures were sold with inadequate U.V. shielding and
you can see how reports of HQI lamps burning corals could come
about. Another interesting possibility is the infrared range. Do
HQI lights put out large amounts of I.R. and could this possibly
damage coral too? This is currently being studied by some hobbyists
(J. Sprung, personal communication). HQI lamps that are not encased
by glass (e.g. Osram HQI lights) must be used in fixtures with U.V.
shielding while bulbs already encased in U.V. absorbing material
(e.g. Energy Savers) should still have some sort of shielding to
protect them from water splashes. Do not be concerned if small
amounts of U.V. are still transmitted, since the majority of
zooxanthellae containing invertebrates require this U.V. to maintain
their U.V. blocking pigments and fluorescent colours.

Coral placement in the tank is another important factor. When putting
new specimens into a tank they should NOT be placed directly under
the HQI light! Many a well- intentioned aquarist has reasoned that
they are rejuvenating their corals after a period of "mistreatment"
in a dealer's tanks. This is similar to running out on the first
sunny day in the spring and lying in the sun for 8 hours hoping to
make up for lost light and vitamin D synthesis deficiency acquired
during the winter. You quickly damage your skin because it has not
had a chance to build up the necessary pigments to protect itself.
The same can occur with corals. The chloroplasts in the zooxanthellae
of corals behave in a similar manner to those found in terrestrial
plants. When faced with lower light levels they produce more
chlorophyll and when faced with too much light they will reduce the
level of chlorophyll. Plants growing under the canopy of a forest
have broader, darker green leaves than the same species of plant
growing in the full sunshine. Corals are no different. If exposed
to lower light levels they become darker brown in colour and their
growth form becomes more elongated and tabular in an attempt to
increase their surface area. Those corals from shallow water tend to
be lighter brown in colour and most have U.V. absorbing pigments that
can, in some cases, give them beautiful fluorescent blues and greens
(Mohan, 1990). By placing corals that have become low-light adapted
or light starved for a period of time, in a brightly lit tank you run
the real risk of light-shock which may damage the coral beyond its
capacity to repair itself.

Another potential problem could be oxygen poisoning (hyperoxyia).
When deeper water corals are placed under brighter light, their
zooxanthellae produce greater amounts of oxygen. Under these high
concentrations oxygen can easily poison both corals and anemones
(Dykens and Shick, 1984; Wilkens and Birkholz, 1986). The response is
often for the coral to quickly expel their zooxanthellae (Wilkens and
Birkholz, 1986). Shallow water anemones cope through various
mechanisms including the use of enzymes to break down oxygen,
withdrawing their tentacles, covering their body column with gravel
to protect it from the sun and to seasonally vary the amount and
ratio of chlorophyll in their zooxanthellae to correspond with
seasonal changes in light intensity (Dykens and Shick, 1984).
Similar behavior may exist in corals. When placing newly acquired
corals into a HQI lit tank, they should be put in the lower regions
away from direct light. Gradually, over a period of a few weeks,
they can be moved closer to the light. Falkowski and Dubinsky (1981)
found that the Red Sea coral, Stylophora pistillata, required 4 weeks
to adapt from shade to light conditions when transplanted. Most
Leather corals (Sarcophyton and Lobophyton) and other soft corals
such as Sinularia sp. can withstand direct HQI light since they
contain high levels of U.V. absorbing pigments and come from
shallower water (Wilkens, 1987). Most hard corals offered for sale
in North America, however, come from deeper water or from shaded
areas and should be gradually acclimated to HQI lights. Shallow
water, reef building species such as Acropora sp. require higher
light levels and can do very well under the right conditions.
However, Dustan (1992) concluded that there may be ecotypes of
zooxanthellae such that those adapted to high light intensities
function poorly in deeper habitats, while deep algae ecotypes are
damaged by the higher light intensities of shallow water. If this is
correct, then certain hard corals that are collected in shaded or
deeper waters, may never be able to adjust to the increased light
intensities of HQI lit aquariums. This may account for the reports of
poor coral behaviour under intense lighting.

Another problem has to do with duration. When putting HQI lights
onto an existing aquarium for the first time, the lights should only
be on for a few hours a day. This time period can gradually be
increased as the animals adapt to the higher light levels. If you
use more than one HQI light they should come on in stages with all
the lights being on for only 2-4 hours and then sequentially turned
off over a period of a few hours. This can gradually be increased to
a maximum of 4-6 hours per day. These recommendations apply also to
high intensity HO and VHO fluorescent lamps.

One thing that I would like to explore is whether the increased light
levels of HQI light increases calcification rates. There are
numerous studies that have demonstrated that calcification rates in
hard corals are dependent on light energy and that light-enhanced
calcification appears to be essential to the construction and
maintenance of coral reefs (Dustan, 1982). It is entirely possible
that, under intense HQI or VHO lighting, the corals may remove
calcium from the water at such a rate that they quickly deplete the
level of calcium ions and begin to suffer. This may be why HQI
lighting, used in conjunction with the regular addition of calcium
hydroxide and strontium chloride solutions (an essential element in
the calcification process) has resulted in greater success in keeping
hard corals in Europe. The lack of such additions, combined with the
other factors described above, may explain the apparent inconsistent
success of HQI lighting in maintaining hard corals in North America.


[Continued in Part 2]