Aquarium LED lighting by PAR: what to stock and spec for planted-tank customers

A retailer once sent us a complaint forwarded from a customer: their new “professional” LED had bleached a carpet of Eleocharis white in under three weeks while a moss wall two tanks over, under the same fixture, melted into algae. Same light, opposite failures. The fixture was not faulty. The buyer had matched the light to the tank’s length and ignored everything that actually governs plant growth: how many usable photons reach the leaf, what colours those photons carry, and how long they arrive each day.

That is the gap this guide closes. If you stock, specify, or build planted aquariums, light intensity is the single specification most often bought on the wrong number. Wattage tells you almost nothing. Lumens tell you what your eye sees, not what a plant uses. The number that matters is PAR at the substrate, read in context with spectrum and photoperiod. Get those three right and a tank is forgiving; get the first one wrong and no amount of fertiliser or CO2 rescues it.

This is written for the trade: shop owners advising customers, aquascapers speccing builds for clients, and wholesale buyers comparing fixtures across price tiers. We cover the physics in plain terms, give working PAR ranges by plant type and by tank depth, explain why WRGB spectrum has displaced cool-white panels, set out a defensible photoperiod, and finish with how to choose a fixture by tank size and when a controller earns its keep.

PAR, PUR, lux, and Kelvin: what each number actually measures

Four terms get thrown around interchangeably on spec sheets and three of them are the wrong tool for choosing a plant light. Sorting them out is the foundation of every decision below.

PAR is the working number

PAR stands for Photosynthetically Active Radiation: the band of light, roughly 400 to 700 nanometres, that plants use to photosynthesise. The figure you see quoted, in units of µmol/m²/s (micromoles of photons per square metre per second, often shortened to “micromols” or just “PAR”), is more precisely the Photosynthetic Photon Flux Density, PPFD. It counts how many usable photons land on a square metre of surface every second (Aquarium Co-Op).

The intuition is distance. Hold the light close to a leaf and PAR is high; push it away, or sink it under more water, and PAR falls. That is why a number printed on the box is meaningless unless you know where it was measured. A fixture rated “120 PAR” at the water surface might deliver 35 at the substrate of a 50 cm tank. Always ask, and always think, in terms of PAR at the substrate, because that is where your carpet and your crypt roots live (Aquarium Boutique).

PAR is the only one of the four metrics that directly describes a light’s suitability for growing plants (American Aquarium Products).

PUR is the quality refinement

PUR, Photosynthetically Usable Radiation, narrows PAR to the wavelengths plants actually absorb most efficiently. Not every photon inside the 400–700 nm window does equal work; chlorophyll a and chlorophyll b have absorption peaks around 430 nm (deep blue) and 662 nm (red), with carotenoids and other pigments filling in elsewhere (2Hr Aquarist). A fixture can post a high PAR figure while wasting much of it in green and yellow wavelengths the leaf reflects rather than absorbs, which is partly why your eye sees green plants as green. PUR is harder to measure and rarely printed, but it explains why two fixtures with identical PAR can grow plants differently. Treat PUR as the reason spectrum matters, not as a number you will shop on.

Lux and lumens measure your eyes, not your plants

Lux and lumens are weighted to the sensitivity of the human eye, which peaks in green. A light tuned to look bright and pleasant to you can be mediocre for plants, and a plant-optimised red-heavy light can look dim while delivering strong PAR (Orphek). Lux is useful for one thing only: a rough, free proxy if you own a lux meter or use a phone app and apply a conversion factor for your light type. It is never a substitute for a PAR reading.

Kelvin describes colour, not power

Colour temperature in Kelvin tells you how warm or cool the white looks: a 6500 K light reads as crisp daylight, 4000 K as warmer, 10000 K as a cold blue-white. It says nothing about intensity and nothing about whether the spectrum suits plants (Charterhouse Aquatics). A 6500 K fixture is a sensible default for a natural planted look, but Kelvin is an aesthetic dial, not a growth specification.

The four metrics at a glance

The one-line takeaway for a buyer: shop on PAR at the substrate, verify the spectrum, and treat lux, lumens, and Kelvin as secondary.

How much PAR your plants need: low, medium, and high light

Plants are sold as “low light,” “medium light,” or “high light,” and those labels translate to PAR ranges measured at the substrate. Sources differ on the exact thresholds because tank depth, CO2, and fertiliser all move the goalposts, but a workable, conservative framework looks like this.

Low light: 10–30 PAR

This covers the hardy, slow-growing species that built the hobby: Anubias, Microsorum (Java fern), Cryptocoryne, mosses, and Bucephalandra. They thrive at the bottom of the range, need only modest fertiliser, and do not require CO2 injection (Aquarium Co-Op). The trade-off is patience: plants grow slowly, but so does algae, which makes a low-light tank the most forgiving build you can sell a beginner. Aquarium Co-Op puts the practical low-light band at roughly 10–20 PAR; others extend it to 30 (The Planted Tank Forum).

Medium light: 30–50 PAR

Most commercially available aquarium plants do well here, including many stem plants, Echinodorus swords, and easier rosette species. Fertilising becomes essential because growth speeds up. CO2 is optional but increasingly worthwhile toward the top of the band, where the difference between a balanced tank and a green-algae bloom narrows (Aquarium Co-Op). This is the sweet spot for a confident hobbyist who wants visible progress without committing to a pressurised gas system.

High light: 50+ PAR

Demanding carpets such as Eleocharis, Glossostigma, Monte Carlo, Hemianthus callitrichoides (HC “Cuba”), and the vivid red stems live here. Above roughly 50 PAR at the substrate, CO2 injection moves from optional to effectively mandatory, and fertiliser dosing must keep pace or algae fills the gap (Aquarium Co-Op). Demanding carpeting plants can want 80–150 µmol/m²/s for fast, tight spread and intense colour (Aquarium Boutique). High light is high reward and low margin for error: it is for clients who will dose, prune, and run CO2, not for a tank that gets attention twice a month.

Working PAR ranges by plant category

A practical note that saves a lot of grief: you do not need to relight a tank to mix plant types. Put high-light plants directly under the fixture and near the surface; tuck low-light species in shaded corners and lower in the column. Low-light plants left in a bright zone tend to grow algae on their leaves, so shading them is the fix, not a lower fixture (Aquarium Co-Op).

PAR by depth: why the same light gives different results

Water absorbs and scatters light, so PAR falls as you descend. This is the mechanism behind the bleached-carpet, melted-moss story at the top of this guide, and it is the single most common reason a fixture rated for “your tank size” disappoints.

The effect is steep. PAR measured at the surface can roughly halve or worse by the time it reaches the substrate of a tall tank, and the loss compounds with anything in the water: surface ripple, tannins, a glass lid versus an open top, dark substrate, shadows from tall plants. Aquarium Co-Op’s own PAR mapping of a fixture over a 65-gallon tank shows the pattern clearly: PAR peaks directly under the centre of the light and drops off toward the corners and toward the bottom (Aquarium Co-Op).

The practical consequences for a buyer:

• A nano cube (20–30 cm deep) reaches the substrate with most of its PAR intact. A modest fixture can easily over-light a shallow tank, which is why even small lights should be dimmable.
• A standard tank (40–50 cm deep) is the design centre of most planted fixtures. A fixture quoting strong mid-tank PAR will usually land in the medium range at the substrate.
• A deep or “tall” tank (55 cm and up) is where buyers go wrong. To put 50 PAR on the substrate of a 60 cm-deep tank, you need a fixture that throws considerably more at the surface, or a more powerful unit than the tank’s footprint alone suggests. Deeper tanks demand stronger lighting, full stop (recifart).

If a customer is choosing between two fixtures for a deep tank, the more powerful, dimmable one is almost always the right call: you can turn a strong light down, but you cannot turn a weak one up. The honest answer to “which light for this tank?” starts with the tank’s depth, not just its length, and ideally ends with a PAR meter reading at the substrate. Where a meter is not available, treat manufacturer surface-PAR figures as a ceiling and assume meaningful loss to the bottom.

Spectrum: why WRGB has displaced cool-white panels

For years, planted tanks ran on banks of cool 6500 K white LEDs. They grew plants and they were cheap. They are now the budget tier for a reason: a white-only spectrum is a compromise that neither maximises plant response nor renders fish and plant colour well.

What plants actually absorb

Chlorophyll a and b absorb most strongly in the red and blue ends of the spectrum, with peaks near 430 nm and 662 nm; green light is largely reflected, which is why foliage looks green (2Hr Aquarist). One commonly cited target for a freshwater plant spectrum leans heavily on red, perhaps 50% or more red (630–700 nm), a substantial slice of green (around 35%, 500–580 nm), and a smaller share of blue (no more than ~15%, 435–495 nm) (aquariumlesson). Red drives growth and red-plant coloration; some green is needed for penetration into lower leaves and for a natural look; blue should be restrained because excess blue is associated with algae and a washed-out appearance.

What WRGB does differently

WRGB fixtures combine White, Red, Green, and Blue emitters. The white base provides broad, efficient coverage and a pleasant natural rendering; the discrete R, G, and B channels let the manufacturer (and, with a controller, the user) tune the mix toward what plants use and toward the colours that make fish and red stems pop (Buce Plant / Chihiros). Compared with a flat white panel, a good WRGB fixture delivers more usable spectrum per watt and dramatically better colour rendering. That second point sells tanks: a red Rotala under a quality WRGB light reads crimson, while under cool white it reads brown-green.

Some premium fixtures add a UVA or violet channel on top of WRGB, marketed to deepen plant coloration and fluoresce certain fish; WEEK AQUA’s P Series, for example, is built around an RGB plus UVA full-spectrum design (WEEK AQUA). The benefit is real for hardscape-and-colour-driven aquascapes; it is a refinement, not a requirement.

Spectrum as an algae lever

Because the colour mix is tunable on RGB-capable fixtures, spectrum becomes a control, not just an aesthetic. Aquarists managing problem tanks deliberately pull back blue and dial in red and white to favour plants over algae (The Planted Tank Forum). That flexibility is the practical case for paying up from a fixed white panel to an RGB or WRGB unit, even before colour rendering enters the conversation.

The buyer’s rule: for any tank where plant health and visual colour both matter, specify WRGB (or RGB) over cool-white. Reserve plain white panels for utility setups and the tightest budgets.

Photoperiod: how long to run the light

Intensity and spectrum decide whether a tank can grow plants; photoperiod decides whether it grows plants or algae. Run an over-bright light too long and you feed algae faster than plants can compete.

A defensible default

For most planted tanks, around 8 hours a day balances plant growth against algae (recifart). For a newly planted tank, start lower, 6 to 8 hours, and let the plants establish before extending, because young plants have not yet built the biomass to outcompete algae (recifart). Avoid running much past about 10–12 hours; extended photoperiods reliably encourage algae (recifart).

The siesta method, and an honest caveat

The “siesta” schedule splits the photoperiod with a midday dark break, for instance four hours on, a three-hour pause, four hours on. The theory is that plants tolerate the interruption and rebuild CO2 during the dark window, while algae, which adapts less readily, is disrupted; recommended siesta gaps run from one to four hours with at least four hours of light on each side (South Scape Aquatica). It is genuinely useful in low-tech tanks without CO2 injection.

Be straight with customers about the evidence, though: experienced aquarists report that a split photoperiod does little for algae or plant growth in many tanks, and its effectiveness is debated (recifart). Treat the siesta as a tool to try, not a guaranteed cure.

Algae troubleshooting via the timer

When algae appears, the timer is the first lever before chemicals. A common fix for filamentous algae is to cut the photoperiod by 30–60 minutes and drop intensity slightly; fast-growing algae thrives on long, bright days and retreats quickly when you trim them (recifart). Pair that with the CO2 and nutrient balance covered in our CO2 guide, because light is one leg of a three-legged stool.

Choosing a fixture by tank size

With the principles settled, here is how to convert a tank into a fixture spec. The honest method is depth-first, length-second, and dimmable-always.

Match length to footprint, then check depth

Manufacturers size fixtures by length so they span the tank and avoid dark corners. A 60 cm light suits a 60 cm tank, a 90 cm light a 90 cm tank, and so on. But length only ensures coverage; depth decides whether enough PAR reaches the substrate. For a shallow scape, a length-matched fixture run at partial power is plenty. For a tank 55 cm deep or more, step up a power tier or accept that the substrate will sit in the low-to-medium band.

Read the wattage and lumen figures as a sanity check, not a target

Wattage and lumens are rough proxies within a single product family, not across brands. As an illustration of how output scales with size and tier, a representative WRGB lineup runs from roughly 33 W and ~2,300 lumens on a 30 cm nano unit up to around 120 W on a 120 cm fixture, with “Pro” variants pushing higher output and a 45 cm Pro listed near 56 W and ~5,000 lumens (Back Water Aquatics; Buce Plant). WEEK AQUA’s P Series PRO V3 scales similarly: roughly 90 W (~5,400 lm) for 60–80 cm tanks, 135 W (~8,100 lm) for 90–120 cm, and 180 W (~10,800 lm) for 120–150 cm (WEEK AQUA / aquascapeshop). Use these to confirm a fixture is in the right class for the tank, then come back to PAR and dimming for the final call.

Always specify dimmable

A dimmable fixture is the buyer’s insurance policy against every uncertainty above: unknown substrate PAR, future plant changes, and algae episodes. You can run a strong dimmable light at 40% for a low-tech tank today and ramp it for a high-tech rescape next year. A non-dimmable light locks the customer into one intensity and one plant palette. For anything but the cheapest utility tank, dimmable is non-negotiable.

A quick fixture-class guide

Controllers: when the app earns its keep

Modern WRGB and RGB-UV fixtures increasingly ship with Bluetooth or Wi-Fi app control. WEEK AQUA’s P Series, for example, controls brightness, spectrum, timers, and sunrise/sunset ramps through its app, with multiple programmable lighting segments across the day for 24-hour automation (WEEK AQUA / aquascapeshop). A controller is worth recommending when:

• The tank is high-tech. Gradual sunrise/sunset ramps and precise scheduling reduce algae and stress, and let you ramp peak intensity only during the CO2 window.
• Spectrum tuning matters. Per-channel control turns spectrum into the algae and coloration lever described earlier. Without it, you are stuck with the factory mix.
• The customer will actually use it. An app is leverage only if it gets touched. For a hands-off owner, a quality fixture on a simple plug timer beats a feature-rich one left at defaults.

For a low-tech tank and a hands-off owner, a non-app fixture on a basic timer is honest advice and saves money. Match the controller to the keeper, not the marketing.

Buyer’s checklist

Before specifying any planted-tank light, confirm:

1. PAR at the substrate, not the surface, is in the band your plants need (low 10–30, medium 30–50, high 50+).
2. Tank depth is accounted for; deep tanks need a power-tier up.
3. The fixture is WRGB or RGB for any tank where plant health and colour matter.
4. It is dimmable, ideally with app or controller scheduling.
5. Length spans the tank to avoid dark corners.
6. A sensible photoperiod (8 hours, less for new tanks) is set from day one.
7. CO2 and fertiliser are planned to match the light level, especially above 50 PAR.

Frequently asked questions

Do I really need a PAR meter to set up a planted tank?
No, but it removes guesswork. PAR meters are expensive for one-off use; many shops own one or rent the service. Without a meter, use manufacturer surface-PAR figures as a ceiling, assume substantial loss to the substrate in deeper tanks, and adjust by observing plant and algae response over a few weeks (Aquarium Co-Op).

Is higher PAR always better?
No. Beyond what your plants and CO2 can use, extra PAR feeds algae, not plants. Light is the accelerator; if CO2 and nutrients can’t keep up, more light just creates an imbalance (Aquarium Boutique). Match light to the rest of the system, and keep it dimmable so you can dial it back.

Can I grow low-light and high-light plants under one fixture?
Yes. Place high-light plants directly under the light and near the surface, and shade low-light species in corners and lower in the column. Low-light plants left in bright zones tend to accumulate algae on their leaves (Aquarium Co-Op).

Why does my light look bright but my plants still struggle?
Brightness to your eye is measured in lux and lumens, which are weighted to human vision and peak in green; they don’t tell you about PAR or spectrum (Orphek). A light can look bright while delivering modest usable photons at the substrate, especially in a deep tank.

What colour temperature should I choose?
Kelvin is an aesthetic choice that doesn’t affect growth on its own. Around 6500 K reads as natural daylight and is a safe default; RGB and WRGB fixtures let you shift the look toward warmer or cooler without changing the underlying plant performance much (Charterhouse Aquatics).

Is the siesta (split) photoperiod worth doing?
It can help in low-tech tanks without CO2, on the theory that plants tolerate a midday break while algae is disrupted (South Scape Aquatica). The evidence is mixed and many aquarists see little effect (recifart). Try it; don’t rely on it as an algae cure.

How do I size a light for a tall (deep) tank?
Start from depth, not length. Water absorbs light steeply, so a 55–60 cm-deep tank needs a fixture that throws far more at the surface to land 50 PAR at the substrate. Step up a power tier and keep it dimmable (recifart).

Are RGB-UV fixtures worth the premium over plain WRGB?
For colour-driven aquascapes, the added UVA/violet channel can deepen plant coloration and make certain fish fluoresce (WEEK AQUA). It’s a refinement, not a requirement; for a function-first planted tank, quality WRGB is enough.

Related articles

• Choosing a CO2 system for a high-tech planted tank
• Aquascaping substrates compared: aqua soil, sand, and inert gravel
• How to dose fertilisers in a planted aquarium without feeding algae
• WEEK AQUA and our aquascaping lighting partners: range overview
• Nano vs. standard vs. deep tanks: matching equipment to footprint

Talk to the trade desk

Innovote Global supplies aquascaping equipment, including planted-tank lighting from WEEK AQUA and other partners, to retailers and aquascapers. If you are speccing fixtures for a build or stocking a range, request a wholesale quote or buy through an authorised channel, and our team will help match PAR, spectrum, and footprint to the tanks you actually keep.

Byline: Innovote Trade Desk