A mixed-reef aquarium creates a difficult lighting problem.
The same tank may contain:
- shallow-water SPS corals,
- lower-light stony corals,
- soft corals,
- zoanthids,
- shaded colonies,
- and corals with very different light histories.
Yet the entire aquarium is usually controlled by one daily lighting program.
That makes questions such as these common:
What time should the lights turn on?
How long should peak intensity last?
How many hours should corals receive light?
Do I need sunrise and sunset ramps?
Should moonlight stay on overnight?
There is no scientifically validated 24-hour schedule that is optimal for every mixed reef.
A better approach is to build the schedule from measurable variables:
PPFD at coral level + duration + total daily exposure + photoacclimation + darkness.
The shape of the graph comes after those decisions.
A 24-Hour Schedule Has Several Different Variables
A reef-light schedule is often shown as one curve, but biologically it contains several separate variables.
Peak PPFD
The highest photon flux the coral receives during the main illuminated period.
Photoperiod
The length of time meaningful daytime illumination is present.
Ramp duration
How gradually the fixture moves between darkness, lower intensity and peak output.
Peak duration
How long the fixture remains near maximum programmed output.
Daily Light Integral
The total PAR photons delivered over the entire day.
Spectrum
How those photons are distributed by wavelength.
Dark period
How long the reef spends under genuinely very low nighttime illumination.
These variables interact, but they should not be confused.
Start With the Coral Positions, Not the Clock
A common approach is:
09:00 sunrise
12:00 peak
18:00 sunset
therefore the schedule is finished.
But this tells us almost nothing about the actual light environment.
The first question should instead be:
What PPFD reaches the different coral positions when the fixture is at its daytime setting?
A single schedule may produce:
300 µmol/m²/s on upper rockwork,
180 µmol/m²/s in the middle,
and:
90 µmol/m²/s on a lower shaded ledge.
Those three corals are simultaneously experiencing different light environments even though the controller shows exactly the same daily curve.
That is why a mixed reef should be treated as a three-dimensional light map.
Do Not Build the Schedule From Fixture Percentages
Suppose an aquarium light says:
Blue 80%
Violet 70%
White 20%
and:
Overall Intensity 65%.
Those percentages cannot tell us what the coral receives.
Actual underwater PPFD depends on:
- fixture power,
- number of fixtures,
- mounting height,
- water depth,
- optics,
- surface movement,
- aquascape,
- coral position,
- and shading.
So a recommendation such as:
Run your reef at 70% for six hours
is not transferable between aquariums.
Controller percentage is useful for reproducing your own setting.
It is not a biological measurement.
There Is No Universal 12-Hour Coral Requirement
A 12-hour day is commonly used in reef aquariums and in coral experiments.
But that does not establish 12 hours as the biological optimum.
Laboratory coral studies have successfully used many different photoperiod designs.
For example, controlled Acropora millepora experiments have maintained colonies under a simple 12 h light : 12 h dark regime.
Another study investigating coral calcification compared natural illumination with a controlled 10 h light : 14 h dark regime and constant-light or constant-dark conditions.
A Goniopora columna experiment directly compared 6-hour and 12-hour illumination periods. Under the blue and violet treatments examined, researchers did not find a general survival advantage from extending illumination from 6 to 12 hours, and several growth measurements were also similar between those durations.
These studies should not be converted into:
6 hours is enough for every coral.
They show something more useful:
Coral photoperiod cannot be reduced to one universal number.
Photoperiod and Intensity Work Together
Consider two lighting programs.
Schedule A
200 µmol/m²/s for 12 hours.
Schedule B
300 µmol/m²/s for 8 hours.
If intensity stays constant:
Schedule A DLI
200 × 12 × 0.0036
= 8.64 mol/m²/day
Schedule B DLI
300 × 8 × 0.0036
= 8.64 mol/m²/day
The total number of PAR photons delivered over the day is the same.
But the biological exposure is not necessarily identical.
Schedule B has a higher instantaneous photon flux.
Schedule A maintains a lower flux for longer.
Coral photosynthesis does not respond linearly to increasing irradiance indefinitely, so equal DLI does not guarantee an identical physiological response. Modeling using 52 published photosynthesis–irradiance relationships and 928 natural diel light curves shows why instantaneous irradiance and integrated daily exposure describe different aspects of reef productivity.
Therefore a useful light schedule considers both:
peak PPFD
and:
DLI.
DLI Is Especially Useful When Comparing Schedules
Daily Light Integral is calculated by integrating PPFD through time.
For constant light:
DLI = PPFD × hours × 0.0036
For a ramped schedule, PPFD changes continuously, so the whole curve must be integrated.
This allows two very different-looking programs to be compared using the same physical quantity.
But there is an important limitation:
There is no universally validated coral DLI table for mixed-reef aquariums.
DLI is best used as a comparison and logging metric, rather than a rigid species prescription.
Why Mixed Reefs Make DLI More Complicated
Imagine that every coral receives the same 12-hour schedule.
At one upper location the peak is:
350 µmol/m²/s
At a middle location:
220 µmol/m²/s
At a shaded lower location:
100 µmol/m²/s
Even if the shape of the curve is identical, daily photon exposure at each location is very different.
That means the aquarium does not have:
one DLI.
It has a spatial DLI distribution.
A Practical Example of a Ramped Schedule
Consider an illustrative schedule with:
3-hour linear ramp up
6-hour plateau
3-hour linear ramp down
followed by:
12 hours of darkness.
This is only an example — not a universal coral recommendation.
At one coral position, suppose the measured peak PPFD is:
250 µmol/m²/s.
During each 3-hour linear ramp, average PPFD is approximately half the peak:
125 µmol/m²/s.
Daily exposure is therefore approximately:
3 h × 125
- 6 h × 250
- 3 h × 125
= 2,250 µmol·h/m²/s equivalent
Then:
2,250 × 0.0036
≈ 8.1 mol/m²/day
So this schedule delivers approximately:
8.1 mol/m²/day
at that particular coral position.
The Same Schedule Produces Different DLI Across the Tank
Using exactly the same curve:
| Measured peak PPFD | Approx. DLI with 3 h ramp + 6 h peak + 3 h ramp |
|---|---|
| 100 µmol/m²/s | 3.24 mol/m²/day |
| 150 µmol/m²/s | 4.86 mol/m²/day |
| 250 µmol/m²/s | 8.10 mol/m²/day |
| 350 µmol/m²/s | 11.34 mol/m²/day |
These are mathematical examples, not coral target recommendations.
The important lesson is that one controller program can create dramatically different daily photon exposures inside the same mixed reef.
Why a Six-Hour Peak Is Not a Biological Rule
You will often see schedules such as:
2 hours sunrise
6 hours peak
2 hours sunset
or:
4 hours ramp
4 hours peak
4 hours ramp.
There is no evidence that six hours at peak intensity is universally optimal.
The correct peak duration depends partly on:
- peak PPFD,
- total daily exposure,
- coral light history,
- species,
- and the rest of the photoperiod.
The peak period is simply one way to distribute photons through the day.
The Ramp Is Optional
The previous AquaHorti article addresses this question in detail:
Does a reef aquarium actually need a sunlight curve?
The answer is:
not as a proven biological requirement.
Corals do possess diel biological rhythms and light-dark cycles clearly matter, but science has not established that healthy coral requires one particular smooth sunrise or sunset curve.
So in this practical schedule guide, ramps should be treated as a tool.
A ramp can:
- create gradual visual transitions,
- extend viewing time,
- change how DLI is distributed,
- and avoid an instantaneous jump to the fixture’s maximum programmed output.
It should not be described as:
the mechanism that wakes the coral up.
A Ramp Does Not Replace Photoacclimation
This distinction is especially important.
Suppose a coral was previously receiving:
100 µmol/m²/s
and the new aquarium eventually reaches:
300 µmol/m²/s.
A two-hour sunrise ramp does not eliminate that threefold difference.
The coral will still spend hours at 300.
If the light change itself is large, the coral may need acclimation over:
days or weeks
rather than a ramp measured in:
minutes or hours.
The separate AquaHorti Coral Light Acclimation article should remain the main reference for that process.
Mixed-Reef Scheduling Is Primarily a Placement Problem
Because a mixed reef contains corals with different light histories and physiological responses, one schedule cannot independently control every colony.
The most effective additional control is:
placement.
Upper areas naturally receive more photon flux.
Lower ledges and partial shading receive less.
That lets the same daily lighting program support different environments.
Instead of attempting to create:
one perfect average PPFD
throughout the tank, create a usable gradient.
Do Not Flatten the Entire Tank to One PPFD
Perfect uniformity sounds attractive.
But a mixed reef actually benefits from having multiple light zones because different corals can be placed according to:
- current exposure,
- species,
- colony history,
- and observed response.
A perfectly uniform aquarium at 250 µmol/m²/s could be less useful than one providing stable zones at several different intensities.
The goal is not:
everywhere equal.
It is:
every coral placed appropriately.
A Practical 24-Hour Example
For a normal display reef, an example schedule might look like this:
| Time | Function |
|---|---|
| 09:00 | Daylight begins |
| 09:00–12:00 | Gradual increase |
| 12:00–18:00 | Main daytime plateau |
| 18:00–21:00 | Gradual decrease |
| 21:00–09:00 | Dark period |
Again:
this is a design example, not an evidence-based universal optimum.
Its advantage is simply that it is easy to:
- measure,
- reproduce,
- calculate,
- and adjust.
Why I Would Not Publish Exact Channel Percentages
The old article title included:
My Favorite 24-Hour Schedule.
That encourages readers to copy settings.
For example:
Violet 90%
Blue 100%
White 25%
That is poor technical guidance because a percentage does not transfer between fixture designs.
The rewritten article should instead say:
Establish the spectrum you want, then measure the resulting PPFD at the coral.
That produces information another person can actually interpret.
How to Adjust the Schedule Scientifically
Once the baseline schedule is established, change the variable that actually needs changing.
If peak PPFD is too high
Reduce peak output.
Do not try to compensate only by shortening the schedule if the coral is being exposed to an unsuitable instantaneous peak.
If the peak appears appropriate but daily exposure is excessive
Reduce:
peak duration
or:
overall photoperiod.
This lowers DLI without necessarily changing the maximum intensity.
If you want a longer viewing period without substantially increasing DLI
Extend the low-intensity shoulders while shortening the high-output plateau.
Then calculate or measure the resulting daily exposure.
If PPFD is suitable but one coral is struggling
Do not automatically redesign the entire tank schedule.
First investigate:
- coral light history,
- placement,
- shading,
- temperature,
- nutrients,
- flow,
- and general health.
One colony is not necessarily evidence that the entire schedule is wrong.
Change One Variable at a Time
This is one of the most useful principles for reef-light troubleshooting.
Suppose you simultaneously change:
- peak intensity,
- blue:white ratio,
- photoperiod,
- ramp duration,
- and coral position.
Then the coral improves.
What caused the improvement?
You cannot know.
A more useful sequence is:
measure → change one variable → allow time → measure again → observe.
This is slower but generates information you can actually reuse.
Do Not Make Daily Adjustments Based on Appearance
Coral fluorescence can change instantly simply because the excitation spectrum changes.
Physiological acclimation takes much longer.
Experiments on variable DLI demonstrate very different acclimation rates between coral species: Pachyseris speciosa showed substantial responses within roughly 3–5 days, while Acropora millepora could require more than 20 days for comparable acclimatory changes.
So changing the lighting every day makes it difficult to determine whether the previous change worked.
Stable Artificial Lighting Is Not Inferior Simply Because It Is Artificial
A useful experiment compared Acropora cervicornis under natural sunlight and controlled LED illumination for 19 weeks.
Linear extension did not significantly differ between treatments, while calcification was actually higher under the LED treatment.
The researchers suggested that differences in spectral composition and more consistent light delivery may have contributed to the different physiological responses.
This does not mean artificial lighting is always better.
It shows that:
successful coral lighting does not require exact reproduction of every natural solar fluctuation.
A Simple Schedule Can Work
Some controlled coral studies use relatively simple light-dark programs rather than elaborate simulated sunrise curves.
For example, Acropora millepora has been maintained experimentally at approximately:
230 µmol/m²/s
under a straightforward:
12 h light : 12 h dark
schedule.
This should not be interpreted as a recommendation to use 230 µmol/m²/s for every Acropora.
The relevant lesson is:
A rectangular photoperiod can support coral physiology; a complex app curve is not automatically superior.
Even Different Photoperiods Can Produce Similar Outcomes in Some Corals
The Goniopora columna experiment is useful here.
Researchers compared 6-hour and 12-hour exposures across several spectral treatments.
Under blue and violet illumination, survival was 100% in both durations, and researchers reported no significant difference in several growth-related comparisons between 6 and 12 hours.
This is not evidence that every Goniopora should receive six hours.
It demonstrates why statements such as:
“Corals require exactly 10 or 12 hours of strong light.”
are too absolute.
The Dark Period Is Part of the Schedule
A 24-hour lighting plan is not only about the hours when the LEDs are on.
Corals possess diel rhythms, and important metabolic and calcification processes continue through darkness.
Research on stony corals has found different gene-expression and calcification patterns during day and night.
Therefore the dark phase should be treated as an intentional part of the program.
For a normal display reef, there is no evidence-based reason to maintain significant visible blue illumination throughout the entire night.
The separate AquaHorti Coral Moonlight and Darkness guide covers that question in more detail.
Do Not Use Moonlight to Extend the “Photoperiod”
Natural moonlight is extremely faint compared with normal reef daytime PPFD.
So if the tank receives measurable photosynthetic irradiance from the supposed moonlight channel for many additional hours, you are not simply creating visual moonlight.
You may be extending daily photon exposure.
For a normal mixed reef, it is clearer to separate:
daylight
from:
optional brief aesthetic viewing light
from:
the main dark period.
What About Feeding Time?
Some corals change polyp expansion through the day-night cycle.
However, this response is species dependent.
A controlled Goniopora columna study, for example, found rapid extension under certain short-wavelength illumination conditions, while broader coral literature shows substantial variation in feeding and expansion behavior among species.
Therefore a mixed-reef light schedule should not be built around the assumption that:
all corals feed only after lights out.
Observe the animals in the particular aquarium.
The Best Schedule Is Reproducible
A good reef schedule should be easy to document.
Record:
- start time,
- end time,
- ramp duration,
- peak period,
- spectrum preset,
- measured PPFD at reference points,
- and any major changes.
Then, if a coral changes over the next month, you can connect the biological observation with a known light history.
Without records, reef-light optimization becomes guesswork.
Build a Reference Map
Choose several permanent measurement positions such as:
Upper reef
Upper-middle
Center
Lower reef
Sand bed
Shaded ledge
Record PPFD at those positions under the daytime peak.
If possible, also record how the measurements change through the programmed curve.
This becomes your aquarium’s light baseline.
Re-Measure as the Reef Grows
A mature reef does not have the same light field it had when it was first aquascaped.
As coral colonies grow, they create:
- self-shading,
- shadows over neighboring colonies,
- changing water flow,
- and new optical geometry.
A location that once received 180 µmol/m²/s may later receive far less.
So a 24-hour program can remain exactly the same while the coral-level exposure changes.
Re-map periodically.
An Example of How to Troubleshoot
Suppose an upper coral is receiving:
350 µmol/m²/s peak
while a lower coral receives:
110 µmol/m²/s.
The schedule uses:
3 h ramp up + 6 h plateau + 3 h ramp down.
Approximate DLI becomes:
Upper position:
350 × 9 equivalent full-intensity hours × 0.0036
≈ 11.34 mol/m²/day
Lower position:
110 × 9 × 0.0036
≈ 3.56 mol/m²/day
If the upper coral has recently been moved from substantially lower exposure, the issue may be acclimation.
If the lower coral appears healthy, there is no reason to increase the entire fixture merely because its PPFD number is lower.
Different positions can intentionally serve different corals.
When to Change Peak Intensity
Consider changing peak intensity when:
- many corals throughout the same zone show a consistent response,
- measurements show the actual exposure differs strongly from what was intended,
- or a major fixture/placement change has altered the aquarium’s light field.
Do not change the entire reef because one coral temporarily retracts.
When to Change Photoperiod
Photoperiod is a useful tool when you want to change total daily exposure without making a large change to peak irradiance.
For example, if a stable coral tolerates the current peak well but you deliberately want to reduce total photons, shortening the plateau can reduce DLI.
Conversely, increasing photoperiod increases DLI even when peak PPFD stays unchanged.
That is why:
“I didn’t change intensity”
does not necessarily mean:
“I didn’t change the coral’s light exposure.”
When to Change Ramp Duration
Ramp duration should mostly be considered in terms of:
- viewing preference,
- distribution of daily photons,
- and avoiding unnecessarily abrupt controller transitions.
It should not be the first variable adjusted when a coral is receiving inappropriate peak PPFD.
A three-hour sunrise does not repair a badly chosen daytime exposure.
A Practical Mixed-Reef Workflow
A repeatable method looks like this:
1. Choose a stable spectrum.
Avoid constantly changing spectral presets.
2. Establish an initial photoperiod.
Use a consistent day-night cycle.
3. Decide whether you want ramps.
Use them if helpful, but do not treat them as mandatory.
4. Measure peak PPFD throughout the reef.
Create a spatial map.
5. Place corals according to their existing light history and response.
Do not rely solely on “SPS / LPS / soft” labels.
6. Estimate or log DLI where useful.
Especially when changing photoperiod or ramp duration.
7. Allow acclimation time.
Do not judge each change the following morning.
8. Maintain a real dark period.
Do not use strong moonlight as an extension of the daytime program.
9. Re-measure after major changes.
Fixture height, aquascape and colony growth all alter exposure.
10. Keep records.
A known history is much more valuable than copying someone else’s schedule.
What Research Supports — and What It Does Not
| Question | Evidence-based answer |
|---|---|
| Do corals need a regular day-night cycle? | Strong biological basis |
| Must the illuminated period be exactly 12 hours? | No |
| Is six hours universally sufficient? | No |
| Does every mixed reef need a sunrise ramp? | No evidence establishes this |
| Can ramps be useful? | Yes |
| Does peak PPFD matter? | Yes |
| Does photoperiod matter? | Yes |
| Does DLI help compare schedules? | Yes |
| Does equal DLI guarantee equal coral response? | No |
| Does one fixture percentage transfer to another fixture? | No |
| Can stable artificial LED lighting support coral growth? | Yes |
| Should a normal reef stay visibly illuminated all night? | Not required |
The Real Purpose of a 24-Hour Schedule
A lighting schedule should not be designed to create the prettiest graph in the controller app.
Its purpose is to control:
when photons arrive
how many arrive
where they arrive
and:
how those exposures change through time.
Once those variables are understood, the curve becomes a useful control interface rather than a biological myth.
Key Takeaway
There is no universal “best 24-hour mixed reef schedule.”
Research shows that reef corals have strong diel physiology, but different coral species can respond very differently to intensity, photoperiod and changing daily light exposure. Variable DLI experiments have demonstrated photoacclimation on timescales ranging from only a few days in one species to more than 20 days in another.
Artificial LED environments can also support strong coral growth without reproducing natural sunlight exactly. In one 19-week Acropora cervicornis comparison, LED-grown colonies showed similar linear extension and greater calcification than colonies exposed to natural sunlight under the experimental conditions.
So instead of copying someone else’s:
09:00 = 20%
12:00 = 100%
18:00 = 60%
schedule, build your own from measurable quantities.
Start with:
coral-level PPFD
then consider:
photoperiod and DLI
then choose:
ramps and viewing periods
and preserve:
a meaningful dark phase.
A good mixed-reef schedule is not the one with the most realistic-looking curve.
It is the one that produces a known, stable and appropriate light environment at the corals themselves.
References
DiPerna, S., Hoogenboom, M., Noonan, S. & Fabricius, K. (2018). Effects of variability in daily light integrals on the photophysiology of the corals Pachyseris speciosa and Acropora millepora. Different coral species showed markedly different photoacclimation rates and growth responses to changing daily light exposure.
Sawall, Y. & Hochberg, E.J. (2018). Diel versus time-integrated (daily) photosynthesis and irradiance relationships of coral reef organisms and communities. Modeling based on 52 published photosynthesis–irradiance relationships and 928 diel light curves demonstrates why instantaneous PPFD and integrated daily exposure provide different information.
Growth, calcification, and photobiology of Acropora cervicornis in natural versus artificial light. A 19-week experiment found no difference in linear extension and greater calcification under LED lighting, demonstrating that exact replication of natural sunlight is not required for successful coral culture.
Evidence for Rhythmicity Pacemaker in the Calcification Process of Scleractinian Coral. Acropora eurystoma exhibited daily calcification rhythms under controlled light regimes, supporting an interaction between internal biological rhythms and environmental light-dark signals.
Effects of LED Light Illumination on the Growth, Digestive Enzymes, and Photoacclimation of Goniopora columna in Captivity. The experiment compared 6- and 12-hour photoperiods under multiple spectral treatments and illustrates why one fixed photoperiod should not be generalized across all corals.