Reef Light Scheduling and Coral Acclimation — How to Ramp Up Without Stress

Why Acclimation Matters

One of the most common mistakes in reef keeping is installing a new light at full power and wondering why corals bleach within a week. Corals contain symbiotic algae adapted to specific light levels. Sudden increases cause photodamage — excess oxygen radicals, bleaching, and potentially death.

Even moving a coral from a lower shelf to a higher shelf in the same tank needs a gradual transition.

New Light Acclimation Schedule

For a new LED over an established tank with existing corals:

Week 1: 30% intensity (it looks dim — that's fine)
Week 2: 40%
Week 3: 50%
Week 4: 60%
Weeks 5–8: Increase by 5–10% per week until target intensity is reached

Watch for: Extended polyps (good), retracted polyps (slow down), colour paling (reduce intensity), tissue recession (drop back to last safe level).

Building a Daily Schedule

A good photoperiod mimics the natural reef day cycle:

Ramp-up (sunrise): 1–2 hours. Blue channels first, then whites join in.
Peak period: 6–8 hours at full target intensity.
Ramp-down (sunset): 1–2 hours. Whites fade first, blues linger.
Moonlight (optional): Very low-intensity blue overnight.
Total light-on time: 10–12 hours including ramps. Many reefers run peaks from around 11am to 7pm for best evening viewing.

Common Scheduling Mistakes

Too many hours — more hours means more algae, not more growth. Reducing peak to 6–7 hours often helps with nuisance algae.
No ramp periods — going from zero to full intensity stresses fish and corals.
Ignoring afternoon sun — if the tank gets direct sunlight, shift the photoperiod or block the natural light.

New Coral Additions

When adding a new coral, place it lower in the tank (lower PAR) and gradually move it up over 2–3 weeks to its final position. It needs time to adjust from whatever lighting it was previously under.

Browse reef lights from UK sellers — when buying used lights, ask the seller what intensity they were running and what corals thrived under it.

What photoperiod are you running and how did you settle on it?