Reef lighting for corals: PAR, PUR and photoperiod

Why coral lighting is a different problem from planted-tank lighting

Our freshwater planted-tank lighting guide covers photosynthesis in true plants, where light tiers span a comparatively narrow range and most beginner species tolerate meaningful variation. Coral photosynthesis happens inside symbiotic zooxanthellae living in the coral's own tissue, and the range of light different coral types need is far wider — from soft corals that thrive in relatively dim conditions to SPS corals adapted to shallow, intensely lit reef zones. Treating "reef lighting" as the same category of decision as planted-tank lighting risks under- or over-lighting demanding species specifically, which is why this guide exists as a separate topic rather than an extension of the freshwater one.

PAR vs PUR

PAR (photosynthetically active radiation) measures total light output in the 400–700 nanometre range a fixture produces. PUR (photosynthetically usable radiation) is narrower: the subset of those wavelengths zooxanthellae can actually absorb and use, concentrated in the blue (roughly 430–470nm) and red (roughly 620–680nm) bands[1]. Two fixtures reporting identical PAR can differ meaningfully in usable output if their spectrum doesn't align well with these bands — part of why coral-specific lighting is generally weighted toward blue rather than a flat, general-purpose white spectrum, and part of why natural reef environments are themselves blue-dominated at depth, since red light is absorbed fastest as water gets deeper[1].

PAR targets by coral type

These bands overlap deliberately — a given LPS species can sit closer to the soft-coral end or the SPS end depending on where it naturally lives — and are widely used specialist-hobbyist starting points[2] rather than a fixed scientific standard. Mismatching a demanding SPS coral with lighting suited to soft corals is a common, avoidable stocking mistake worth checking before buying, not after growth stalls.

A fish-only marine tank with no coral doesn't need to meet any of these PAR ranges at all — lighting there is a viewing and fish-welfare choice, not a photosynthesis requirement, and there's no reason to run reef-intensity lighting just because the tank is saltwater. A mixed-reef tank keeping soft, LPS and SPS corals together generally solves the mismatch through placement rather than through a single compromise intensity: SPS higher up and closer to the light, softer corals lower down or in partial shade from rockwork, so each coral sits closer to its own target range within one fixture's overall output rather than every coral receiving the same PAR regardless of what it actually needs.

What affects how much light your coral actually receives

The PAR ranges above describe light at the coral, not the raw output printed on a fixture's box, and several things separate the two. Placement inside the tank matters most day to day: a coral positioned higher, closer to the light, generally receives more PAR than the same coral placed lower or shaded by rockwork, so "target PAR for this species" is really a placement decision as much as a fixture-buying one. Fixture mounting height above the water surface changes intensity too — the same light mounted higher spreads its output over a wider area at lower intensity per point, while mounting it lower concentrates more PAR onto less of the tank. Water clarity affects how much of that light reaches any given depth: significant particulate matter, degraded filter media, or a tank overdue a water change can measurably reduce light penetration compared with clear water, which is one reason a coral that was doing fine can start struggling after water quality drifts even if the fixture itself hasn't changed. Spectrum interacts with all of this: blue-weighted output is generally favoured for reef-specific fixtures both because it matches PUR absorption better than a flat white spectrum and because it's widely reported to bring out coral colouration more vividly — a genuine aesthetic preference in the hobby, not only a photosynthesis-driven one.

Photoperiod: how long reef lights are commonly run

Most reef tanks run a total photoperiod of roughly 8–11 hours a day[2], but there's no single correct number that applies to every reef, and treating any figure in that range as a fixed rule is a mismatch waiting to happen. A tank running high-intensity SPS-appropriate lighting is often kept toward the shorter end of that range, since the coral is receiving a lot of PAR per hour it's lit; a tank running gentler soft-coral or LPS-appropriate intensity can often run somewhat longer without over-exposing anything, because total exposure is a product of intensity and duration together, not duration alone. That's the key relationship worth holding onto: duration can't be judged independently of intensity. Running a weak or underpowered fixture for longer hours doesn't reliably substitute for genuinely adequate PAR at coral placement, and stretching the photoperiod to compensate for low intensity is a common but unreliable fix — if PAR at the coral is genuinely too low, the fixture or its positioning is the thing to address, not the clock.

"Photoperiod" itself is also not one flat number in practice. Most schedules distinguish a total illuminated period (lights active at any brightness, including the dim ramp at either end) from a shorter peak-intensity period sitting inside it — commonly, intensity is ramped up gradually over roughly one to two hours, held at or near peak for several hours in the middle of the day, then ramped back down over a similar period rather than switching on and off abruptly[2]. A tank reported as running a "10-hour photoperiod" might only be at genuinely peak intensity for six or seven of those hours, with the rest spent ramping — worth knowing when comparing your own schedule against something you've read elsewhere, since two tanks quoting the same total hours can differ meaningfully in how much of that time is spent at full brightness. Gradual ramping mirrors natural dawn/dusk transitions more closely than an instant on/off cycle and is widely reported to reduce stress on both coral and fish compared with sudden lighting changes, though this is general hobby practice rather than the subject of a dedicated study on ramping itself.

Fish-only, soft-coral, LPS, SPS and mixed-reef tanks don't need the same treatment here either. A fish-only marine tank can run essentially any photoperiod that suits viewing and a normal day/night rhythm for the fish, since there's no coral photosynthesis to consider. A soft-coral or LPS-dominant tank running lower PAR has more flexibility on total duration than an SPS-heavy tank running near the top of its intensity range, where a longer total exposure at high intensity is more likely to compound into a bleaching-risk situation (see the link to bleaching below) than the same duration would at soft-coral intensity. A mixed-reef tank generally follows whichever coral in it is least tolerant of a long, intense exposure, then relies on the placement approach described above to keep less-demanding corals comfortable within that same schedule.

Two example starting schedules — starting frameworks to adapt to your own tank and fixture, not universal prescriptions:

• Soft-coral/LPS-leaning tank: roughly 1.5-hour ramp up, 5 hours near peak, 1.5-hour ramp down — around 8 hours total illuminated period, toward the lower-PAR end of the ranges above.
• SPS-heavy tank: roughly 1-hour ramp up, 4 hours near peak, 1-hour ramp down — around 6 hours total illuminated period, kept shorter specifically because intensity is running much higher.

Both are starting points for a new setup or an uncertain schedule, meant to be adjusted from there once you've watched how your own tank responds — not a fixed target every tank running that coral type should converge on.

A note on moonlight settings: a genuine simulated moonlight, run at very low brightness overnight, is a different thing from an extended photoperiod, and it's worth keeping the two separate in how you think about your schedule. If a "moonlight" setting is bright enough to be doing meaningful photosynthetic work, it isn't functioning as a dark period at all — it's effectively extending the illuminated period into the night, undoing the genuine dark period corals and fish would otherwise get, without necessarily showing up in how you'd normally describe your schedule's total hours. When totalling up your tank's real illuminated period, an overly bright moonlight setting belongs in that count, not treated as separate from it.

Change one variable at a time

Lighting, lighting duration, coral placement, and even unrelated changes like a new dosing routine can all affect how coral looks and behaves within the same rough window of time, which makes it easy to misattribute a change to the wrong cause if you alter more than one thing at once. A simple checklist for any lighting-related change:

• Change intensity or duration — not both — in the same adjustment. If you're not sure which one needs correcting, see "should I change intensity or duration first?" in the FAQ below.
• Make one fixture, placement or schedule change at a time, not several in the same week, so a response you see afterward has one plausible cause rather than three.
• Wait long enough to actually see a response before changing anything else — polyp extension can shift within days, but colour and growth take weeks; changing a second variable before the first has had time to show its effect makes both impossible to interpret.
• Keep a record of what changed and when, since "I think I changed the schedule around the same time" is a much weaker basis for a decision than an actual dated note.
• Don't change lighting and add new coral, adjust dosing, or alter flow in the same session — each is a plausible explanation on its own for anything you observe afterward.

Setting a schedule and changing it gradually

For a new tank, a new fixture, or simply a schedule you're unsure about, starting conservative is the lower-risk direction: a shorter total photoperiod and an intensity toward the lower end of the range for your coral mix, with room to extend either one later once you've watched how your specific tank responds. It's far easier to add ten more minutes of light or a modest intensity increase next week than to recover from weeks of over-exposure, and unlike PAR at the coral, duration is something you can adjust immediately with no equipment change required.

Changing an established schedule — in either duration or intensity — works the same way acclimating a new coral to a fixture does: gradually, not in one step. A common approach is adjusting total duration by small increments, for example 15–30 minutes at a time, no more than every few days to a week, watching how coral responds before making the next change, rather than moving from an 8-hour to an 11-hour photoperiod overnight because a schedule you read about online used a bigger number. The same caution applies to shortening a photoperiod in response to an algae problem (see ambient light and algae below) — a sudden, large cut is no more appropriate than a sudden, large increase.

Acclimating coral to new lighting

A coral's zooxanthellae adjust their pigment density and photosynthetic machinery to whatever light level they've been living under — whether that's a fixture in your tank or the depth they were collected or propagated at. A sudden jump to significantly brighter lighting, whether from a new fixture or moving coral to a brighter spot, can overwhelm that adjustment before the zooxanthellae have time to adapt. This applies across three genuinely different situations, and it's worth being explicit that all three need the same gradual treatment even though they don't always get thought of the same way:

• A new fixture, or upgrading an existing one. Run it at reduced intensity for roughly one to two weeks and increase gradually, rather than switching straight to full target output on day one — the whole tank is affected at once here, which is what makes this the highest-stakes version.
• Increasing intensity on an existing fixture (a new bulb, a firmware/output change, or simply turning the dial up), even without changing hardware. The coral hasn't changed, but the light reaching it has, and the same gradual increase applies.
• Relocating coral within the tank — moving a piece from a shaded lower position to a higher, more exposed one, or into a tank with a different fixture entirely (new purchase, frag swap, or your own aquascape rearrangement). This is easy to overlook, since nothing about the lighting itself changed, but the coral's actual light exposure did. Move newly relocated coral to an intermediate position for a week or two before its final placement where practical, rather than straight to the brightest spot in the tank.

In all three cases, the standard approach is the same: reduced intensity or exposure initially, increasing gradually over roughly one to two weeks, giving the coral's zooxanthellae time to build up appropriate photoprotection rather than being shocked by the full target intensity immediately. This is specifically about intensity; see setting a schedule and changing it gradually above for the equivalent, equally gradual approach to changing total duration, and change one variable at a time above for why combining any of these with another change at once makes a bad response hard to interpret.

What to watch for in your coral

Coral gives some feedback on whether a lighting setup suits it, though none of it is a precise instrument reading and all of it needs interpreting alongside everything else going on in the tank. Polyp extension is one of the more useful day-to-day signals for many soft and LPS corals — polyps that stay open and extended for a normal portion of the day generally indicate a coral that's comfortable enough to feed and photosynthesise normally, while polyps that stay retracted or closed for unusually long periods are worth investigating, though retraction has several possible causes beyond lighting (a nearby coral stinging it, a water-quality issue, recent handling, or simply a normal response around feeding or at night for some species) and isn't proof of a lighting problem on its own. Colour is a slower, less immediate signal — a coral losing colour or visibly paling over days to weeks can reflect zooxanthellae under light or temperature stress (see the link to bleaching below), but colour can also shift for reasons unrelated to lighting, and a single day's appearance tells you very little either way. Growth is the slowest signal of all, measured in weeks and months rather than days, and a coral that isn't growing isn't necessarily under-lit — nutrient availability, water chemistry stability and coral health generally all affect growth rate too. Treat all three as things worth noticing and investigating rather than as standalone proof that a lighting change specifically was the cause of whatever you're seeing.

Ambient light and algae

A tank's total light exposure isn't only whatever the main fixture provides on its own schedule. Ambient daylight from a nearby window, or a room light left on well outside the tank's own photoperiod, adds to that exposure and can make a carefully set schedule less controlled in practice than it looks on paper — worth accounting for by keeping the tank out of direct window light where practical, or treating a consistently bright room as part of the tank's effective photoperiod rather than ignoring it.

Algae growth appearing or increasing is often assumed to mean the photoperiod is too long, but algae in a reef tank — like algae in any aquarium — tracks available nutrients as much as it tracks light, and usually more so. Elevated phosphate or nitrate from feeding, stocking, or infrequent water changes, alongside insufficient herbivorous clean-up crew, is at least as commonly behind a spreading algae problem as excess lighting, and cutting the photoperiod without checking or addressing nutrient levels often doesn't resolve it. Algae alone isn't evidence that photoperiod specifically is the cause; it's a prompt to check nutrients and husbandry first, and to treat a lighting change as one possible factor among several rather than the default explanation.

When a PAR meter helps

A PAR meter measures light intensity at a specific point — wherever you hold the sensor — which makes it genuinely useful for confirming what a coral is actually receiving at its exact placement, rather than relying on a fixture's rated output or a generic distance-from-light estimate. It's particularly useful for checking PAR at coral placement as a fixture ages, since LED output commonly declines gradually over years of use, and for re-checking after moving coral, rearranging rockwork, or changing water clarity meaningfully. What a single PAR reading can't do by itself is tell you whether a fixture's spectrum is well-matched to PUR, diagnose why a specific coral looks unhappy, or substitute for watching the coral itself over time — a reading confirms intensity at one point at one moment, not overall lighting quality or coral wellbeing.

The link to bleaching

Common mistakes

• Buying an SPS coral for a tank lit for soft corals, or the reverse mismatch, without checking PAR compatibility first.
• Upgrading to a brighter fixture and running it at full intensity immediately, without an acclimation period.
• Judging spectrum by eye alone rather than checking a fixture's actual PAR/PUR output where data is available.
• Ignoring placement depth when comparing a coral's stated PAR requirement against your fixture's rated output at the glass.
• Increasing light intensity during a heatwave or equipment fault, compounding two bleaching-risk factors at once.
• Running lights longer to compensate for a weak or underpowered fixture, rather than checking PAR at coral placement and addressing intensity directly.
• Cutting the photoperiod as a first response to algae without checking nutrient levels and husbandry, which are at least as commonly the actual cause.
• Running a "moonlight" setting at high brightness overnight rather than a genuinely dim simulated moonlight, effectively extending the photoperiod without realising it.
• Jumping straight to a new total duration or intensity read about elsewhere, instead of changing an established schedule gradually.

Frequently asked questions

How AquaPulse can help

A lighting upgrade or a new coral addition is a specific, dateable event — but its effect on existing coral, good or bad, unfolds over the following weeks, not immediately. Knowing whether a coral that's looking less vibrant this month is responding to last month's lighting change, or something else entirely, depends on being able to place that change on a timeline against everything else you've logged, not on memory of roughly when you swapped the fixture.

It won't tell you a lighting change caused a colour shift — nothing can do that from the outside, and AquaPulse doesn't try. What it does is give equipment changes and coral notes a shared timeline alongside your water testing, so when you're trying to work out what happened last month, you're reading a record instead of reconstructing one from memory. Downloads are on the App Store and Google Play; Water Check covers the logging feature itself.

Sources and uncertainty

Good to know: This guide provides general informational guidance only. It does not recommend a specific brand or product, and PAR/PUR figures are widely used approximations rather than a guarantee for any specific fixture, coral species or tank.

• [1] Reefco Aquariums — "PAR Levels for Corals (SPS, LPS, Soft Corals)" Specialist reef-hobbyist reference source. Used for the PAR/PUR distinction and the blue/red-wavelength absorption bands zooxanthellae use most efficiently. Not a scientific authority; presented as widely reported hobby consensus.
• [2] Top Shelf Aquatics — "SPS Coral Lighting: PAR, Spectrum, and Photoperiod Explained" Specialist reef-hobbyist reference source. Used to independently cross-check by-coral-type PAR ranges and commonly used photoperiod/ramping convention. Not a scientific authority; presented as widely reported hobby consensus.
• [3] Journal of Experimental Marine Biology and Ecology — "Photoinhibition, bleaching susceptibility and mortality in two scleractinian corals... in response to thermal and light stresses" Peer-reviewed primary research. Used for photoinhibition as a documented mechanism linking high light (particularly combined with elevated temperature) to coral bleaching risk. This is specialised coral-physiology research, not aquarium-specific guidance; we've kept our use of it narrow and qualified accordingly.

What we deliberately left out: a precise numeric PAR ceiling above which bleaching is guaranteed, since the cited research describes photoinhibition as one contributing mechanism among several and doesn't establish a single aquarium-relevant threshold; species-by-species PAR figures for every commonly kept coral, since individual tolerance varies enough within each broad category (soft/LPS/SPS) that a single number per species would overstate precision the evidence doesn't support; a single "correct" photoperiod in hours, since intensity, coral mix and individual tank goals all affect what's appropriate; a fixed ratio or exact number of minutes for ramp-up and ramp-down, since fixtures and preferences vary; and a claim that a longer photoperiod produces more coral growth, since growth depends on several factors together and running lights longer at the same intensity isn't a reliable way to increase it.

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