Reef Tank Photoperiod: How Many Hours to Run Your Lights, Plus PAR, Spectrum and Fixtures

Lighting is the piece of reef equipment people spend the most on and understand the least. This is the long version: what PAR actually measures, why spectrum changes what you see more than what your coral does, how long to run your lights, and how to work out how many fixtures your tank really needs.

There is a particular kind of frustration that shows up in every reef forum. Someone buys a good light, plumbs a good tank, keeps their parameters clean — and their corals just sit there. Not dying. Not growing. Browning slowly over six months while the owner doses harder and tests more often. Nine times out of ten it is light. Either not enough of it reaching the coral, too much of it landing on a coral that wanted shade, or the right amount delivered on a schedule that never lets the animal settle. The good news is that lighting is one of the few reef parameters you can actually measure precisely, fix once, and largely stop thinking about. You just need to know what you are measuring.

Part 1: PAR, and why watts mean nothing

PAR stands for Photosynthetically Active Radiation. It counts the photons landing on a surface that fall within the 400–700 nanometer band — the range photosynthetic organisms can actually use. It is measured in micromoles per square meter per second, written µmol/m²/s, which everyone shortens to “PAR” in conversation. Here is what matters about that definition: PAR counts usable photons at a specific point in your tank. Not how bright the fixture is. Not how much power it draws. Not how bright it looks to you. Which is why the numbers on the box are close to useless:

• Watts measure electricity consumed, not light produced. A 150W fixture from 2010 and a 150W fixture from today are not remotely comparable.
• Lumens measure brightness weighted for the human eye, which peaks in green-yellow. Corals care most about blue. A light optimized for lumens is optimized for the wrong species.
• Kelvin describes the color appearance, not the intensity. A 20,000K light is not stronger than a 10,000K one. It is bluer.

Two fixtures with identical wattage can deliver wildly different PAR at your sandbed. The only number that means anything is the one measured in your water, at the spot the coral is sitting.

What your corals actually want

Coral light requirements are a range, not a target, and the ranges overlap more than beginners expect. Most corals are far more adaptable than the charts suggest — what they cannot tolerate is a sudden change.

Water eats light faster than you think

The single most under-appreciated fact in reef lighting: PAR falls off roughly with the square of distance, and water absorbs on top of that. The reading at your sandbed is a fraction of the reading just under the surface — and the drop is steepest in the first few inches.

Two practical consequences fall out of this curve. Tank depth changes everything. A standard 24" tall display loses a huge fraction of its surface PAR by the time light reaches the sand. Deep tanks need genuinely more powerful fixtures, not just more of them. Your rockwork is a light gradient. This is a feature. The top of your aquascape is your SPS shelf, the middle is LPS, the shaded overhangs and sandbed are for mushrooms and low-light LPS. Building the scape with that gradient in mind gives every coral a home without touching the dimmer.

Part 2: Why a PAR meter is the best money you will spend

Everything above is theory until you measure your own tank. And here is the thing — you genuinely cannot eyeball PAR. Your eye adapts constantly and is heavily biased toward the white channel. Two tanks that look identically bright can differ by 150 PAR. A meter turns lighting from guesswork into a five-minute job. Take readings on a grid across your tank, at the depths where corals actually sit, and you learn things you cannot get any other way:

• Where your hot spot is. Nearly every fixture has one, usually dead center. That is where your Acropora goes — and where a newly-arrived chalice will bleach.
• Where the dead corners are. Tank corners routinely run at a third of center readings. That explains the one coral that never colors up.
• Whether you need another fixture, or just need to raise the one you have.
• What your intensity slider actually does. The relationship between percentage and output is rarely linear.
• Whether your LEDs have aged. Diodes decline slowly. A five-year-old fixture at 100% may deliver what it used to at 70%.

How to map your tank properly

1. Set your lights to full daytime peak — the intensity they actually run at mid-photoperiod, not 100% if you never use 100%.
2. Work a grid, not random spots. Nine points across the footprint is a sensible minimum on a standard tank.
3. Measure at coral height, not at the surface. Readings at the water line are meaningless for placement.
4. Keep the sensor flat and horizontal. Tilting it changes the reading substantially.
5. Write it down. A simple sketch of your tank with numbers on it is a reference you will use for years.

Then place corals to the map rather than to the aesthetics, and let the aesthetics follow.

Part 3: Spectrum — blue, white, and what your eye is doing to you

Spectrum is where reef lighting gets genuinely interesting, and where most of the arguing happens. Corals are animals, but they run on solar power borrowed from an algae. Zooxanthellae — the symbiotic algae living inside coral tissue — photosynthesize and hand most of that energy to the host. Their chlorophyll absorbs most strongly in the blue end of the spectrum, and to a lesser extent in the red. Blue also happens to be what survives on a real reef. Water absorbs red wavelengths within the first few meters. At 10 meters down on a Caribbean reef, red is essentially gone and the world is blue. Corals evolved under that light.

Why reef tanks look so blue — and why that is not just fashion

Blue-heavy light does three things at once. It matches what zooxanthellae absorb best. It matches the light corals evolved under. And it excites the fluorescent proteins that make corals glow those improbable greens, oranges and pinks. That last one is worth understanding, because it explains why the same coral looks completely different in two tanks. Fluorescent proteins absorb short wavelengths — violet and blue — and re-emit them at longer wavelengths. Feed a coral a lot of violet and its greens light up. Wash it in white light and the fluorescence is still happening, but it is swamped by reflected white and you simply cannot see it. So when someone says a light “makes corals pop”, they mean it is violet-heavy. That is a viewing preference and a legitimate one.

The color versus growth trade-off

Here is the honest version of a debate that gets overheated. Broad-spectrum light with meaningful white content tends to produce slightly faster growth. Blue-heavy light tends to produce better color, because corals under intense blue produce more protective pigment — and pigment is color. Most reefers land somewhere in the middle: blues at or near full, whites dialed to somewhere between 10% and 40% depending on taste. Every major manufacturer ships a preset that does roughly this, and those presets are genuinely well-designed. If you are not sure, run the manufacturer preset. It is a better starting point than a spectrum you invented.

Part 4: Photoperiod — how long to run the lights

Length of day is the cheapest lever in reef lighting and the one people fiddle with most and understand least. More hours is not more growth. Past a point, extra hours mostly grow algae. The consensus that has emerged over years of reef keeping is a ten-hour photoperiod, structured as one hour ramping up, eight hours at peak, one hour ramping down.

Getting the schedule right

• Consistency beats optimization. Put it on a timer or a controller and stop touching it. A schedule that shifts around confuses the animal more than a slightly imperfect one held steady.
• Longer is not better. Fourteen-hour photoperiods do not grow coral faster. They grow algae faster and give your pH swing a wider arc.
• Blue-only dawn and dusk is a nice touch. Many controllers can run blues for 30–60 minutes either side of the main period. It looks superb and eases the transition.
• Actinic “moonlight” all night is unnecessary and can disrupt spawning behavior and fish rest. Give the tank real darkness.
• Do not run lights longer to compensate for a weak fixture. Intensity and duration are not interchangeable. A coral needing 250 PAR will not get there on 120 PAR for twice as long.

Part 5: The technologies — LED, T5, metal halide and hybrids

All three technologies still grow beautiful coral. Anyone who tells you otherwise is selling something. What differs is spread, running cost, controllability and the shape of the light.

LED

Now the default, and for good reason: fully controllable intensity and spectrum, programmable ramps, twenty-plus year diode life, low running cost and no bulb replacement. Modern reef LEDs cluster many diode colors to build a spectrum you can tune channel by channel. The historic knock on LEDs was shadowing. Light from a small point source travels in fairly straight lines, so a branching Acropora shades its own lower branches, and corals tucked behind rock get less than the meter suggests. Manufacturers have largely solved this with diffusers, wider optics and multi-puck designs — but it remains the reason SPS growers still love T5.

T5 fluorescent

Long tubes producing broad, diffuse, even light across the whole footprint. T5 has almost no shadowing, and SPS tanks under T5 have a distinctive even coloration that many long-time reefers still consider the gold standard. The costs are real: bulbs need replacing every 9–12 months as output degrades, there is no dimming or ramping without specialist ballasts, they generate more heat, and you cannot tune spectrum except by choosing bulb combinations.

Metal halide

The old king. A single intense point source that produces genuine shimmer — the dancing caustics on the sand that no LED quite replicates — and enormous penetration into deep tanks. If you have a 30" deep display, halide still makes a compelling argument. Against it: heat output that often demands a chiller, high electricity draw, annual bulb replacement, and no dimming. Halide is now a niche choice, but a legitimate one.

Hybrid fixtures

T5 tubes plus LED pucks in one housing. You get LED controllability and color-tuning with T5 fill light killing the shadows. For a mixed or SPS-dominant display this is arguably the best light money can buy, and the reason hybrids remain popular despite costing more than either technology alone.

The brands worth knowing

A quick orientation to who makes what, without pretending there is one correct answer.

Part 6: How many fixtures does your tank need?

The question everyone asks, and the answer that annoys everyone: it depends on the fixture, because coverage varies enormously between designs. A wide-diffuse fixture covers far more footprint than a dense point-source puck of the same wattage. That said, you need somewhere to start. These are sensible starting points for a mixed reef — add output if you are SPS-dominant or running a tank deeper than 24".

Mounting height matters more than people expect

Mounting height is really a question about beam angle. Light leaves a fixture as a cone, and that cone has to have widened enough to cover your whole footprint by the time it reaches coral height. Raise the fixture and the cone widens — but intensity falls off with roughly the square of the distance, and the portion of the cone that now overshoots the glass is simply lighting your floor. That tension is why no universal number exists. A fixture with narrow optics throws a tight, intense cone and has to sit higher to spread it. A bar-style array is already wide and can sit close. Two fixtures of identical wattage can want mounting heights a foot apart, purely because of the lenses sitting in front of the diodes. Bulk Reef Supply tested roughly fifty fixture-and-height combinations over a 24" cube and published the results, which are worth reading in full. The spread they found is the useful part: about 7" suited a Radion XR15 Pro, 10" a Red Sea ReefLED 90, 14" an AI Hydra 64, and around 20" a five-bar Orphek array. They judged a height acceptable when the center reading stayed within 30% of the tank average and at least 70% of the light still landed inside the glass — two sensible thresholds you can apply to any fixture you own. The practical version: start around 8–10" above the water for most puck-style LEDs, then measure. If your center reading is enormously higher than your corners, raise it. If you are lighting the carpet, lower it. This is another job the meter does in ten minutes.

Part 7: Acclimation, and the mistakes that cost corals

You can get every number above right and still bleach a tank by moving too fast. The same applies to new corals. A frag arriving from a vendor has been growing under a specific intensity, which is rarely yours. Place new arrivals lower in the tank than their eventual home and walk them up over a few weeks. This is also why aquacultured corals settle in more predictably than wild-collected pieces. A coral grown in a controlled system under stable light has never known anything else, so the change it experiences on arrival is smaller. Everything we ship is grown in our own systems in Key West under consistent reef lighting.

The mistakes we see most

• Buying on wattage. It tells you about your electricity bill and nothing about your corals.
• One fixture on a four-foot tank. Covered above. Two moderate units, every time.
• Chasing someone else’s settings. A screenshot of another reefer’s schedule is meaningless without their fixture, their mounting height and their tank depth.
• Adjusting constantly. Corals need weeks to respond. If you change the spectrum every two weeks, you are permanently acclimating and never growing.
• Blaming light for a nutrient problem. Brown, dull corals in a tank with high nitrate and phosphate are usually telling you about nutrients, not photons.
• Never replacing T5 bulbs. They keep lighting up long after their useful spectrum has degraded. Replace on schedule, not on failure.
• Ignoring the glass. Salt creep and a dirty lens can eat a surprising amount of output. Wipe the fixture down.

The short version

1. Measure, do not guess. PAR is the only number that matters, and it has to be measured in your water at coral height.
2. Build a light gradient into your rockwork and place corals to it: SPS high, LPS mid, softies low and shaded.
3. Run blues near full, whites to taste. Manufacturer presets are a good starting point.
4. Ten hours, one up, eight peak, one down. Then leave it alone.
5. Two moderate fixtures beat one powerful one on anything over three feet.
6. Start new lights at 30–40% and ramp over a month.

Get those six right and lighting stops being the thing you worry about. Which frees you up to worry about alkalinity instead.

The Underwater Gardener grows aquacultured corals in Key West, Florida (FL Aquaculture Certificate #AQ0254072). Our corals are raised under stable reef lighting in our own systems, so they arrive already adapted to a well-run tank. Corals ship UPS Next Day Air every Tuesday and Wednesday.