First and foremost, most corals can be placed into three major categories: Stony (exterior skeletons), Soft (no calcium-based skeleton), and SPS (Short/Small Polyp Stony). Naturally, the scientific world can categorize these animals down to the "nth" degree, but for our purposes, these three categories will suffice.
When it comes to how corals grow, each category of animal requires its own unique lighting requirements, but all share one common trait; photosynthesis in order to survive. Just as plants convert sunlight to produce chlorophyll, marine animals survive similarly by converting light energy into "food". Actually, this energy is consumed by zooxanthellae algae that produce by-products that the corals need to survive; a truly symbiotic relationship.
How Changes in Lighting Effects Corals
The coloration of coral polyps and tissues is dictated by these zooxanthellae. By differing the spectral output of our tank lighting we can actually influence the ultimate color/shading of our corals. How? Let's say that we have been using a 5500K VHO fluorescent setup at 220 watts. We get that itch to spend money and help our tank and animals by installing a 250-watt metal halide with a 10,000K lamp. Aside from the aesthetics of the rippling light show these lamps provide, we have suddenly changed the frequency of light that all the animals in our system have grown accustomed to. We use the term "frequency" to describe the change in CRI or spectral output.
Often, the corals will shrink up, close their polyps, or otherwise show their displeasure at this sudden and drastic change in their energy source. It's actually the billions of symbiotic algae that are recoiling, sending shock waves through their host and causing this rapid change in appearance. Within days, and sometimes even hours, the zooxanthellae will adapt to this new frequency and intensity by changing their absorption capabilities or their overall color. That's right, the coral's color is actually that of their hitch-hiking algae, adapting to the increases or decreases of ultraviolet and other energy-source factors.
Have you ever looked at a Tridacna clam from the top of the tank, then lower your gaze to a sideways view, only to be disappointed? Strange how dull the clam's color appears from the side, while from above all those rich and vibrant colors seem to shout at the sky? Well, that's the clam's zooxanthellae algae, doing their thing, protecting the clam's delicate tissues from sunburn!
Whenever a change in lighting takes place, and don't be fooled, even replacing old tubes/lamps with the exact same wattage and URI can create the same response if the old tubes have been allowed to degrade past their useful spectral output, the system should be allowed to gradually adjust to this major change. How? We usually replace lamps and tubes just after the system shuts down for the night. I then replace or exchange old for new and ensure that next morning, not all the lights kick on at the same time, allowing intervals between pairs of types of tubes. If you have only a two-tube system this isn't possible, but the installation of a dimmer circuit like ones found on some electronic ballasts makes the chore much easier to accomplish.
Remember that corals and their zooxanthellae adapt to changes in their surroundings the same as we humans do. Whereas we may shade our eyes from bright sunlight, these animals have no such luxury. They must react as only they can, by recoil and a slow but gradual return to normal behavior. Interesting how we can't discuss lighting without getting involved in the coral's actual physical properties, isn't it? Well, after all, it IS the primary reason for lighting at all!
About Lighting for SPS Corals
SPS (Short/Small Polyped Stony) corals are by far the most numerous in the skeleton category. We will not delve into the physiology or other biological factors of these corals, other than to state that they, above all others, require the most dynamic of light sources.
Not until the advent of aquarium related halide lamps was this light source truly ready for our use. The incredible intensity of the metal halide lamp makes providing the right output of light ideal for maintaining these delicate-to-establish corals. Once they grab hold, SPS corals can be the most prolific of all their kind, growing at enormous rates and prompting many cuttings. These coral cuttings, known as frags, can then be propagated through "coral-farming", which is highly practiced by many hobbyists and commercial aquaculture companies today.
Naturally, factors other than the lights themselves contribute to the success of any coral, but once the water parameters and the lighting system are acceptable to the animals themselves, watch out.
