Lighting for Corals in Saltwater Aquariums

Saltwater reef habitat

SPS corals and the importance of correct CRI and intensity have been discussed previously. To begin, most corals may be divided into three categories: Stony (skeletons on the outside), Soft (skeletons without calcium), and SPS (Short/Small Polyp Stony). Naturally, scientists can classify these species to the "nth" degree, but these three groups will sufficient for our needs.

When it comes to how corals develop, each type of animal has its own set of illumination needs, but they all have one thing in common: photosynthesis. Similar to how plants convert sunlight into chlorophyll, marine organisms survive by transforming light energy into "food." Actually, algae use this energy and make by-products that the corals require to thrive; it's a true symbiotic interaction.

How Changes in Lighting Effects Corals

These zooxanthellae determine the color of coral polyps and tissues. We can truly impact the eventual color/shading of our corals by varying the spectrum output of our tank lights. How? Let's pretend we've been utilizing a 220 watt 5500K VHO fluorescent system. Installing a 250-watt metal halide with a 10,000K bulb satisfied our desire to spend money and aid our tank and creatures. Aside from the beauty of the rippling light display provided by these lights, we have abruptly changed the frequency of light that all of the creatures in our system have become accustomed to. The shift in CRI or spectral output is referred to as "frequency."

Corals frequently shrink, shut their polyps, or otherwise express their dissatisfaction with this abrupt and extreme change in their energy supply. The billions of symbiotic algae recoil, generating shock waves through their host and creating the quick change in appearance. The zooxanthellae will adjust to the changed frequency and intensity within days, if not hours, by modifying their absorption capacity or overall hue. The color of the coral is really that of their hitch-hiking algae, which adapts to changes in UV and other energy-source parameters.

Have you ever been disappointed after looking at a Tridacna clam from the top of the tank and then lowering your eyes to a sideways view? Strange how the clam's hue seems drab from the side, yet all those rich and brilliant colors cry at the sky from above? That's the clam's zooxanthellae algae at work, shielding the clam's sensitive tissues from the light.

When a change in illumination occurs, the system should be allowed to gradually acclimate to this substantial change. Don't be deceived; even replacing old tubes/lamps with the exact same wattage and URI can provide the same response if the old tubes have been allowed to decay past their usable spectrum output. How? After the system has been turned off for the night, we normally replace the bulbs and tubes. I then replace or swap the old tubes with new ones, making sure that not all of the lights turn on at the same time the next morning, providing for gaps between pairs of tube types. This isn't possible if you just have a two-tube system, but installing a dimmer circuit, such as those found on certain electronic ballasts, makes the job much easier.

Remember that corals and their zooxanthellae, like people, respond to changes in their environment. We have the luxury of shading our eyes from intense sunshine, but these creatures do not. They must react in the only way they know how: by recoiling and gradually returning to normal behavior. Isn't it interesting how we can't talk about illumination without going into the physical qualities of the coral? After all, that is the basic reason for lighting in the first place!

About Lighting for SPS Corals

In the skeleton category, SPS (Short/Small Polyped Stony) corals are by far the most common. We won't go into detail on the physiology or other biological aspects of these corals, except to say that they, more than any other, demand the most dynamic light possible.

This light source was not completely suitable for human usage until the introduction of aquarium-related halide lights. The metal halide lamp's tremendous intensity makes giving the correct amount of light excellent for preserving these difficult-to-establish corals. SPS corals, once established, can be the most prolific of all corals, growing at incredible speeds and producing numerous cuttings. These frags, also known as coral cuttings, can subsequently be propagated through "coral-farming," a popular hobby among amateurs and professional aquaculture firms today.

Other aspects, aside from the lights, have a role in the success of any coral, but if the water parameters and lighting system are acceptable to the critters, be on the lookout.