Coral Lighting: How Changes in Lighting Affect Corals

In this guide

Lighting is the single most important piece of equipment in a reef tank, and it is also the one most likely to cause visible harm if changed carelessly. Corals adapt to a given light source over weeks, and any sudden change to intensity or spectrum can undo that adaptation quickly.

Why corals are so light sensitive

Reef building corals host zooxanthellae, a photosynthetic algae living inside their tissue, and derive a large part of their energy from what that algae produces. This relationship works within a fairly narrow window. Too little light and the coral cannot generate enough energy; too much light and the coral is forced to expel the algae as a stress response, a process better known as bleaching.

What bleaching actually looks like

• Colour fading to a pale or pastel shade, often the first visible sign, before more complete whitening sets in.
• Whitening that starts at the highest, most exposed parts of the coral and spreads downward, since those areas receive the most light.
• Persistent polyp retraction, especially on the side of the coral facing the light source.
• Tissue that remains physically intact even as colour is lost, unlike the tissue loss seen with bacterial infections or predation.

Common lighting mistakes that trigger stress

• Upgrading to a new, more powerful light fixture and running it at full intensity immediately, rather than ramping it up gradually.
• Moving a coral from a shaded, lower part of the tank straight into a high light zone without a transition period.
• Removing carbon or GFO media, which clears the water of tannins and other compounds and allows noticeably more light to penetrate than the tank has had recently.
• Increasing the white channel intensity or photoperiod too quickly on a controllable LED fixture.

The role of spectrum, not just intensity

Blue wavelengths in the 420 to 470 nanometre range are the primary driver of coral photosynthesis and are also what stimulates the fluorescent pigments that give many corals their vivid colours. This is why reef tanks are typically lit with a blue dominant spectrum rather than the more neutral white light used over fish only tanks, and why simply adding more white light does not automatically make corals more colourful.

Temperature makes corals more sensitive to light stress

Light is not the only factor at play. On natural reefs, bleaching is triggered mainly by elevated water temperature, and the same holds true in a tank: a coral sitting in slightly warm water is far more likely to bleach at a given light intensity than the same coral in water at its normal target range. This matters in India, where ambient room temperatures through summer can push a tank above 28 to 29 degrees Celsius if the chiller or cooling fans are undersized. If bleaching appears after a lighting change during hot weather, check the tank temperature alongside the lighting settings rather than assuming light is the only cause.

How to change lighting safely

• Introduce any new fixture at a fraction of its full intensity, and increase output gradually over two to three weeks.
• Acclimate newly purchased corals to your existing lighting the same way, starting them lower in the tank or under partial shade before moving them to their permanent spot.
• Change one variable at a time. Adjusting intensity, spectrum and photoperiod together makes it impossible to tell which change caused a coral's reaction.
• Keep a simple PAR reading or fixture setting log so you have a record to fall back on if something changes unexpectedly.

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