Light Scheduling Checklist for Reef Keeping

Your coral mix determines the safe PAR range and overall intensity ceiling. Soft corals and many LPS often thrive around 50-150 PAR, while SPS systems commonly target 200-350 PAR in upper rockwork.

New tanks under 3-6 months old usually do better with more conservative intensity and shorter peak periods to limit nuisance algae and avoid stressing newly added frags. Mature, stable reefs can handle stronger and more consistent output once nutrients and chemistry are predictable.

A practical starting point is 8-10 hours of total daylight with a 4-6 hour high-intensity peak. Running full power for 10-12 hours often increases algae pressure without improving coral growth.

Assign high-light zones for Acropora and Montipora, moderate zones for most LPS, and low-light areas for mushrooms, zoanthids, and shaded chalices. This helps you program one schedule that still supports multiple coral types through placement.

Hot spots directly under LEDs and dim corners near the glass create uneven growth and color. Before finalizing the schedule, verify that rock shelves, islands, and sandbed areas receive the intended PAR for the corals placed there.

A bluer spectrum can improve fluorescence and visual pop, while a broader spectrum with strong blue and controlled white often supports more natural daytime appearance and efficient growth. Knowing the goal prevents constant schedule changes that stress corals.

Ramping up over 1-2 hours reduces sudden light shock compared to instant full output. This is especially important for LED systems, which can jump to high PAR immediately if not programmed carefully.

Most reefs respond well to a shorter high-energy period rather than blasting maximum light all day. A focused peak supports photosynthesis while reducing bleaching risk and excess film algae on glass and rock.

A smooth ramp-down helps fish settle naturally and avoids abrupt darkness that can startle wrasses, tangs, and other active species. It also gives a more realistic transition for coral feeding response in the evening.

For most reef tanks, this range balances coral energy needs with manageable nutrient demand. If nutrients are ultra-low and corals look pale, reducing intensity may be better than extending the photoperiod.

Bright moonlights left on all night can disrupt fish rest and provide little benefit to corals in a home aquarium. If you use them for viewing, keep them extremely dim and limit the run time.

Running a refugium light on a reverse cycle can help soften nighttime pH drops, especially in heavily stocked systems. This does not replace display light scheduling, but it can improve overall daily stability.

Do not rely only on manufacturer percentages because mounting height, rockscape, water clarity, and screen tops all change delivered light. Use a PAR meter to confirm real values in the exact spots where corals sit.

Many Acropora, Stylophora, and birdsnest corals color and grow best in this range when nutrients and alkalinity are stable. Pushing much above 350-400 PAR without careful acclimation can bleach sensitive frags.

Hammer, frogspawn, acans, favia, and many blastos often do well in moderate light with steady flow. Too much PAR can cause tissue recession, shrunken polyp extension, or washed-out coloration.

Discosoma, Rhodactis, some ricordea, and shaded non-demanding corals typically fare better in lower zones or under overhangs. Overexposure often leads to shrinking, bleaching, or moving off the rock.

Most successful reef schedules place the majority of usable output in the 400-470 nm range, where zooxanthellae photosynthesis is efficient and coral fluorescence is strongest. This also lets you maintain visually pleasing coloration without relying on excessive white light.

High white settings can make the tank appear brighter than the PAR actually needed, while excessive red and green often contribute little to coral performance and can highlight nuisance algae. Keep these channels restrained unless you have measured reasons to increase them.

If PAR is too concentrated directly under the fixture, lifting the light can smooth distribution and reduce harsh hot spots. This is often safer than simply lowering percentages and leaving poor spread uncorrected.

Even if the tank looked fine under the old light, different optics and spectrum profiles can dramatically change coral stress. Start lower and rebuild the schedule gradually over 2-4 weeks.

Most modern LED controllers allow a slow ramp to the final program over several weeks. This feature is one of the easiest ways to prevent bleaching after upgrades or major percentage changes.

Freshly shipped corals are already stressed from transport, temperature swings, and handling. Starting them on the sandbed or in partial shade for 5-14 days helps them settle before they are exposed to final placement intensity.

These are common early signs that the light increase was too aggressive. If they appear, reduce intensity or shorten the peak period immediately rather than waiting for full tissue loss.

Corals tolerate change poorly when multiple stressors stack at once. If alkalinity is drifting more than about 0.3-0.5 dKH day to day, stabilize chemistry before pushing PAR higher.

Clear lenses and fresh bulbs can noticeably increase delivered light, sometimes enough to stress corals that were adjusted to older output. Treat maintenance-related output jumps as real schedule changes, not minor housekeeping.

Corals often need time to respond to spectrum and intensity changes, and constant tweaking makes it harder to identify the true cause of problems. Stability is often more valuable than chasing a perfect-looking preset.

Salt creep and dust reduce PAR and can distort spread over time. A fixture that seems underpowered may simply be dirty, especially over open-top reef systems with strong surface agitation.

T5 output and spectrum shift as bulbs age, even if they still turn on and look bright to your eyes. Staggering bulb replacement can reduce sudden changes if the fixture is a major part of your total lighting.

Pale SPS can be caused by too much light, but also by very low nitrate and phosphate, such as nitrate under 2 ppm or phosphate under 0.02 ppm. Brown corals may reflect excess nutrients, low flow, or insufficient PAR rather than spectrum alone.

Older fixtures, enclosed canopies, and poorly ventilated hybrid setups can raise daytime temperature enough to stress corals. Aim to keep daily temperature variation tight, usually around 77-79 F with minimal spikes.

Documenting changes helps you connect outcomes like improved polyp extension, bleaching, or algae outbreaks to a specific timeline. This makes future tuning more precise and prevents repeating the same mistakes.