Lighting: The Energy That Powers Corals

Back in Lesson 1 you met the partnership at the heart of the reef: corals house tiny algae that photosynthesize and feed them. Light is the fuel for that engine. This lesson is about providing it well, which, like flow, is less about maximum power and more about the right conditions kept stable.

Why this matters for your tank

For most of the corals you’ll keep, light is their primary food source. Too little and they slowly starve, losing color and failing to grow. Too much, too suddenly, and they bleach, expelling their algae in stress. Lighting is one of the biggest levers on coral health, and one of the easiest to get subtly wrong, so it’s worth understanding rather than guessing.

Core concepts

Corals eat light (mostly)

The zooxanthellae inside coral tissue turn light into sugars and share them with the coral. This is why lighting is non-negotiable for photosynthetic corals, and why a coral going pale is a warning sign: it’s losing the very algae that feed it. A minority of corals (like some non-photosynthetic species) feed entirely by catching food, but as a beginner you’ll keep light-dependent corals, so treat light as a meal.

Spectrum: why reef tanks look blue

Water absorbs red and yellow light quickly with depth, so corals evolved under blue-dominant light. Reef fixtures lean heavily blue for two reasons: it’s what corals are adapted to photosynthesize under, and it makes their fluorescent pigments pop. You’ll see this expressed as color temperature (Kelvin): higher K is bluer. A blue-rich look isn’t just aesthetic; it’s biologically appropriate.

Intensity: enough, not maximum, measured by PAR

Reefers measure usable light intensity as PAR. Different corals want different amounts:

• Soft corals and LPS: lower PAR, often placed lower in the tank.
• SPS: higher PAR, usually placed up high near the light.

You don’t need a PAR meter on day one (many local clubs will lend one), but the concept matters: intensity should match the coral, and it drops off fast with depth and distance. This is why coral placement (height and shading) is part of lighting.

Photoperiod and stability

Keep a consistent daily schedule: many reefers run 8–10 hours of full light with gentle ramp-up and ramp-down at the ends to mimic sunrise and sunset. Put it on a timer or a programmable LED so it never varies. As with chemistry and flow, corals value consistency: a stable, moderate light regime beats an intense but erratic one.

Acclimate new corals

A coral coming from a dim shipping bag or a different tank can’t handle full light immediately. Acclimate it: start it lower in the tank or dim the lights, then increase over a week or two. Skipping this is a common cause of a new coral bleaching within days of purchase.

In your tank

• Buy a quality reef LED or T5 fixture rated for your tank’s depth. Reef lighting is not the same as freshwater or general aquarium lighting.
• Put the lights on a timer and keep the schedule identical day to day.
• Place corals by their light needs: low-light corals low and shaded, high-light corals up top.
• Acclimate every new coral by easing it into full intensity over 1–2 weeks.
• Change bulbs/settings gradually. A sudden jump in intensity stresses everything, even if the new level is “correct.”

You’ve now built the environment: stable water, good flow, appropriate light. The stage is set for the life itself. The final module starts with the star of the show: coral biology, and what these strange animals actually are.