Light Is Food

Return to the first chapter's central fact: your coral farms photosynthetic algae, and those algae eat light. That reframes your reef light from decoration into a feeding appliance. When you set light intensity, spectrum, and duration, you are literally rationing your corals' food supply. Get it right and corals grow and glow. Get it wrong in either direction — too little or too much — and they starve or burn.

This chapter turns light from a mystery into three dials you can set with intention: how much, what color, and how long.

PAR: How Much Light

PAR — photosynthetically active radiation — measures the amount of usable light reaching a specific spot, in units called PPFD. It is the number that matters, because lux and watts tell you about the fixture, while PAR tells you about the coral. And PAR falls off fast with depth and distance, so the reading at your water surface is meaningless; what counts is the reading where each coral actually sits.

Here are working target ranges, measured at the coral:

• Soft corals: roughly 50 to 150 PAR. Low light, lower third of the tank, shaded corners.
• LPS corals: roughly 75 to 200 PAR. Moderate light, middle rockwork and sand bed.
• SPS corals: roughly 200 to 400 PAR. Strong light, upper rockwork nearest the fixture.

These overlap because corals adapt, and because individual species vary. Treat them as starting zones, not laws. The single most useful accessory you can borrow or buy is a PAR meter, because without one you are guessing at your corals' food ration. Many local reef clubs lend one out; an afternoon mapping your tank's PAR is worth more than a month of forum speculation.

Why More Light Is Not More Growth

Beginners assume that if some light is good, more is better. It is not. Corals adapt their zooxanthellae to a light level, and once the algae are saturated, extra light does nothing useful — and beyond a point it causes harm. Excess light triggers the coral to expel algae in self-defense, which is bleaching, and it drives corals to brown up or pale out depending on the stressor.

Growth is limited by whatever is scarcest, and in most tanks that is chemistry or flow, not light. A coral bathed in intense light but starved of stable alkalinity will not grow faster; it will just sit there, or slowly decline. Add light only when you have confirmed light is the limiting factor, which for softies and LPS it rarely is.

Spectrum: What Color of Light

Corals evolved under water that filters out warm colors with depth, leaving the blue end of the spectrum dominant on a reef. Their zooxanthellae and pigments are tuned to that blue-heavy light, which is why reef tanks look blue and why that look is functional, not fashionable.

Spectrum drives coloration as much as growth. The bright pigments corals produce — the greens, reds, and electric blues that make a reef pop — fluoresce and intensify under blue and violet wavelengths. A coral that looks muddy brown under warm white light can reveal stunning color under a blue-heavy spectrum, because you are exciting those pigments. This is why a good full-spectrum reef LED lets you tune individual color channels: you are balancing enough full-spectrum light for growth against enough blue and violet for color and for the pigment response corals show off. Over a nano tank, a capable nano reef LED light delivers the same blue-heavy balance in a smaller package sized for softies and LPS.

A caution on chasing color: pushing spectrum toward extreme blue makes corals fluoresce beautifully but can under-feed the zooxanthellae that need broader wavelengths. Health first, then color. A coral that is growing and open under a balanced spectrum will always look better long-term than one held pale under blue-only light.

Photoperiod: How Long

Corals do not need marathon lighting. A photoperiod of eight to ten hours of full intensity, bracketed by ramp-up and ramp-down periods that mimic sunrise and sunset, is plenty. Longer does not mean more growth; it mostly means more algae on your glass and rock.

The ramps matter more than beginners expect. Corals, like their algae, do not enjoy being slammed from darkness to full noon intensity. A gentle 30-to-60-minute sunrise and sunset reduces stress, and it is one reason programmable LEDs earn their price. Set a modest peak period with generous ramps and leave the schedule alone — consistency beats tinkering.

Acclimating Corals to Your Light

Here is where new keepers lose corals without realizing why. A coral arriving from a store or another tank was grown under a specific light. Drop it straight under your intense fixture and it can photo-shock and bleach even if your PAR is technically correct for its type. The coral is not adapted to your light yet.

Acclimate deliberately. Place new corals low in the tank, in the shade, for the first week or two, then move them up gradually over several weeks to their target zone. Alternatively, if your light is programmable, run it at reduced intensity and ramp it up over two to three weeks. Either way, you are giving the zooxanthellae time to adjust their density to the new light level. This single habit prevents a huge share of new-coral bleaching, and it costs you nothing but patience — the currency this whole hobby runs on. The demanding end of this, naturally, is SPS, where photo-acclimation is not optional.

Putting the Three Dials Together

A sensible starting setup for a mixed reef: a blue-heavy full-spectrum light, an eight-to-nine-hour peak photoperiod with 45-minute ramps, intensity set so the top of the rock hits LPS-to-SPS PAR and the bottom hits softie PAR. Place corals by their light needs — SPS up top, LPS in the middle, softies below and in the shade — and acclimate every newcomer low and slow. Set it, confirm it with a PAR meter once, and then resist the urge to keep fiddling. Light problems in established tanks almost always come from changing light, not from the light itself.

Signs Your Light Is Wrong

Corals report on their lighting if you know the signals. A coral receiving too little light slowly browns as its zooxanthellae multiply to catch what light there is, loses its bright pigments, stretches or leans toward the source, and eventually stops growing. Move it higher or increase intensity gradually.

A coral getting too much light does the opposite: it pales or bleaches as it sheds algae in self-defense, may take on a washed-out or bright-white look that is one step from tissue loss, and closes up or retracts its polyps. Move it lower or reduce intensity, and do it gradually rather than in one jump, since a sudden drop stresses corals too.

The trap is that browning and paling can both read as simple color change, so interpret them alongside behavior and position. A browning coral that is also stretching toward the light needs more; a paling coral that is also closing up needs less. When in doubt, change one thing, wait a week, and read the response. Because light trouble in established tanks nearly always traces back to a change you made, the fix is usually to stop adjusting and let the corals re-adapt to a stable setting.

FAQ

What PAR levels do corals need?

As a working guide measured at the coral: soft corals want roughly 50 to 150 PAR, LPS corals about 75 to 200, and SPS corals about 200 to 400. These ranges overlap because corals adapt and species vary, so treat them as placement zones rather than exact targets, and confirm with a PAR meter rather than guessing from the fixture's wattage.

Can too much light hurt corals?

Yes. Beyond the level a coral's algae can use, extra light provides no benefit and can trigger bleaching, in which the stressed coral expels its zooxanthellae and pales. Excess light also drives corals to lose color rather than gain it. Growth is usually limited by chemistry or flow, not light, so adding light rarely speeds growth and often causes harm.

How do I acclimate a new coral to my lights?

Place the new coral low in the tank, in shade, for its first week or two, then move it up toward its target light zone gradually over several weeks. If your light is programmable, you can instead reduce intensity and ramp it back up over two to three weeks. This lets the coral's algae adjust to your light and prevents the photo-shock bleaching that kills many newly added corals.