Photoperiod and Acclimation: Programming Your Reef Lights Correctly

Lighting intensity gets all the attention, but photoperiod and proper acclimation protocols are equally important for coral health. Getting these right prevents bleaching and maximizes growth.

What Is Photoperiod?

Photoperiod refers to the daily duration of the light cycle. For reef tanks, this means the period when your lights are at or near full intensity, plus the ramp-up and ramp-down periods.

Recommended Photoperiod

Total Light Duration

• 9–12 hours total photoperiod is appropriate for most reef systems
• 10 hours is the most common recommendation for mixed reef tanks
• Less than 8 hours may limit growth; more than 14 hours can encourage nuisance algae

Ramp-Up and Ramp-Down

• Simulate natural sunrise and sunset with 1–2 hour ramp periods
• Starting a ramp from 0% blue, increasing to white and full intensity
• Ramp-down mirrors ramp-up in reverse

Example 10-Hour Schedule

7:00 AM – 8:00 AM: Ramp from 0% to 50% (dawn)

8:00 AM – 6:00 PM: Full intensity (midday)

6:00 PM – 7:00 PM: Ramp from 50% to 0% (dusk)

7:00 PM – 11:00 PM: Moonlight only (1–2% blue)

Acclimating Corals to New Lighting

Lighting changes are one of the most common causes of coral bleaching. Whether you're upgrading fixtures, moving corals, or starting a new tank, acclimation is essential.

New Tank / New Fixture Acclimation Protocol

Week 1–2: Run lights at 30–40% intensity

Week 3–4: Increase to 50–60% intensity

Week 5–6: Increase to 70–80% intensity

Week 7+: Reach target intensity; monitor coral response

Moving Corals to Higher Light

1. Place coral at the lowest light position in the tank initially
2. Allow 2–4 weeks for the coral to demonstrate full polyp extension and normal coloration
3. Gradually raise position over the following weeks
4. If polyps close or tips bleach, move the coral back down immediately

Signs of Light Stress

• Brown-out (too low PAR or too much): Zooxanthellae density increases, coral browns
• Bleaching (too high PAR or too fast): Zooxanthellae expelled, coral whitens
• Polyp retraction (often high PAR): Polyps stay withdrawn during the high-intensity period

Dimming and Spectrum Control

Modern LED fixtures allow independent control of different color channels. Recommended settings:

• Blue (420–470nm): The most important channel; should be 70–100% of maximum
• White/full-spectrum: 20–50% provides natural coloration without washing out blue
• Red: Low levels (5–15%); too much red encourages nuisance algae
• UV/violet: Beneficial for coral fluorescence; 20–40%

Conclusion

Consistent photoperiod, gradual ramp-up/down, and patient coral acclimation prevent the majority of light-related coral losses. Take 6–8 weeks when introducing new fixtures or moving corals to significantly higher PAR.

Disclaimer: This content is intended for educational purposes only. Information may change as reefing practices evolve and new data becomes available. First Due Aquatic Rescue is not responsible for outcomes resulting from the use of this information. Always verify guidance across multiple reputable sources.

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